66 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



blast; the inner one, the entoderm, or hypoblast; the two constituting 

 the primary germinal layers of the embryo. Excepting among the lowest 

 forms of the Metozoa, there is subsequently formed between these a third 

 layer, the mesoderm (mesoblast) (Figs. 39, 40). 



Gastrulation as described above occurs in a number of different sec- 

 tions of the animal series; but in many instances, owing to a relatively 

 large amount of yolk present in the eggs of some animals, it may become 

 modified in various ways so that it is doubtful if in some of these cases 

 true gastrulation occurs. 



Cases in which the blastula is a hollow ball of cells, one hemisphere 

 apparently dimpled into the other (Fig. 40), are represented by the eggs 



bp 



Fig. 39. Fig. 40. 



Fig. 39. — Section. Mesoderm formation {mes). (bp) Blastopore, (ect) Ectoderm. 

 (ent) Entoderm. 



Fig. 40. — Sagitta (arrowworm). Folds developing from the bottom of the gastrula 

 giving rise to the definitive mid-gut (enf) and the coelomic diverticula (cod), (ect) Ecto- 

 derm, (ejit) Entoderm, (som. m) Somatic mesoderm, (splm) Splanchnic mesoderm. 

 (From Hertwig.) 



of the arrowworms (Sagitta), many moUusks (Paludina, etc.), sponges, 

 coelenterates, worms, echinoderms, and Amphioxus. In this process of 

 invagination (emboly) the outer layer forms the ectoderm; the inner 

 layer, the entoderm. In the medusa Lucernaria where a morula is 

 formed, the cavity being absent, the cells of the animal pole (ectoderm) 

 grow over those of the vegetative pole (entoderm). This process of 

 overgrowth is known as "epiboly." A modification of this type is found 

 also in the yolk-laden eggs of birds and reptiles with discoidal cleavage 

 wherein the germ disk overgrows the yolk. In some coelenterates a 

 layer of cells is formed inside the blastula by delamination, the outer 

 layer then constituting the ectoderm, the inner one the entoderm. 



The mesoderm has a variable origin. It is represented in the coelen- 

 terates and sponges by a gelatinous material, the mesogloea, which 

 appears between ectoderm and entoderm; and into this, cells wander 

 from the two layers. In other Metazoa, the middle layer may arise 

 from a few primary mesoblasts or cells which appear at an early stage 



