Formation of the Germ Disc
Either during gastrulation or after it has been completed, superficial cells assemble to form the germ rudiment, which represents the starting point for the formation of the adult body organization (Figure 6.1). As has been shown in amphipods, this process can involve dramatic cell migration (Scholtz and Wolff 2002). In crustaceans with yolky eggs, this phase is characterized by the germ disc that marks the future ventral side of the embryo (Figure 6.1 A). The germ disc comprises outer ectodermal cells and inner layers or groups of mesodermal, endodermal, and primordial germ cells (e.g. Samter 1900; Manton 1928; Weygoldt 1958; Browne et al. 2005). These cells are free of yolk and densely packed. In some cases, vitellophages are situated in the yolk mass, which digest the yolk (Scholtz and Wolff 2002). The remaining area of the egg surface is called the extraembryonic region, because it plays no formative role. In this region, the cells are large, contain yolk, and are loosely arranged (Figure 6.1 A, B). Probably some of these cells also act as vitellophages. With the formation of the germ disc, the anteroposterior axis of the future body becomes recognizable (Figure 6.1 A). The germ disc elongates and turns into the germ band. Characteristic landmarks are the anterior and posterior invaginations of the stomodaeum (ectodermal mouth and foregut) and the proctodaeum (ectodermal anus and hindgut) (Figure 6.IB). At the germ band’s anterior margin, the head lobes (Scheitelplatten of Cladocera; see Samter 1900; Kiihnemund 1929) form, which are the anlagen of the lateral eyes and some anterior brain parts (Manton 1928; Kiihnemund 1929; Weygoldt 1958; Scholl 1963; Dohle 1972; Browne et al. 2005; Mittmann et al. 2014) (Figure 6.1 B). At the posterior end, the pre-anal growth zone is differentiated (Ooishi 1959, 1960; Dohle 1970; Scholtz 1992) (Figures 6.IB
FIGURE 6.1 Formation of the germ disc and the transformation into the germ band. (A) The early germ disc of the amphipod Cryptorchestia garbinii. The future anterior end of the embryo is on top. Blastoderm cells getting rid of their yolk migrate together forming the germ disc (gd), whereas loosely arranged cells form the extraembryonic region (eer). (B) Ventral view (anterior on top) of the early germ band showing an egg nauplius of the crayfish Astacus asta- cus with the advanced anlagen of the eyes (head lobes, hi), the first and second antennae and the mandibles (al, a2, md) compared with more posterior segmental structures. At the anterior, there is the anlage of the labrum (lr) covering the forming stomodaeum, in the posterior there is the caudal furrow (cf) and the forming caudal papilla (cp) with the proctodaeum (modified after Reichenbach 1886). (C) SEM of an advanced embryo of the amphipod Gammarus pulex (lateral view, anterior to the left, dorsal on top). Here, an egg nauplius is lacking because the naupliar region shows no advanced development. The naupliar appendages are in a similar stage as in the crayfish in B, but in contrast to this the fifth thoracic segment (th5) shows early limb buds. (D) Lateral view of the embryo of an isopod (lateral view, anterior to the left, dorsal on top). The germ band shows no caudal furrow but stretches along the ventral side. In contrast to the situation shown in В and C, a dorsal furrow (df) is formed. Some thoracopods (thp) are visible. (E) The nauplius larva of the northern krill Meganyctiphanes norvegica. This is a reduced non-feeding nauplius with few cells (nuclear fluorescent stain bisbenzimide).
and 6.2A-E). This produces the ectodermal and mesodermal cellular material that elongates the germ band and eventually gives rise to the ectodermal segmental structures such as the intersegmental furrows, ganglia, limbs, and the mesodermal musculature. During this process, the endodermal midgut precursor cells form the midgut and the midgut glands, if present (Reichenbach 1886; Fioroni 1970). With the elongation of the germ band and the differentiation of the segmental structures, the germ band extends laterally and the extraembryonic region gets smaller until the lateral halves of the embryo meet at the dorsal side (Figure 6.1C, D). This dorsal closure is accompanied by the digestion of the yolk mass, which at the hatchling stage is reduced to a large degree or entirely gone (Scholl 1963).
However, these stages are frequently overlapping and not strictly separated. Furthermore, a proper germ disc and the subsequent germ band are absent in species with small, almost yolk-free eggs that undergo total or mixed cleavage such as some copepods, cladocerans, and malacostracans (e.g., Kuhn 1913; Fuchs 1914). In these cases, the germ anlage comprises the entire embryo including the dorsal side (Figure 6.IE).