28 



CLEAVAGE AND THE GEEM LAYERS 



THE FORMATION OF THE ECTODERM AND ENTODERM (GASTRULATION) 



The blastula and early blastodermic vesicle show no differentiation into 

 layers. Such differentiation takes place later in all vertebrate embryos, giving 

 rise first to the ectoderm and entoderm, and finally to the mesoderm. From these 

 three primary germ layers all tissues and organs of the body are derived. 



The processes of gastrulation, by which ectoderm and entoderm arise, and of 

 mesoderm formation will be treated separately. 



Amphioxus and Amphibia.— In these animals the larger cells at the vegetal 

 pole of the blastula either fold inward, i. e., invaginate (Amphioxus, Fig. 19), or 

 are for the most part overgrown by the more rapidly dividing cells of the animal 



B C 



Fig. 19. — Gastrulation of amphioxus (Hatschek in Heisler). X220. A, Blastula: a, animal cells; 

 V, vegetative cells; c.c, cleavage cavity. B, Beginning invagination of vegetative pole. C, Gastrula, the 

 invagination of the vegetative cells being complete: ect., ectoderm; ent., entoderm; arch., archenteron; 

 bl, blastopore. 



pole (amphibia) . Eventually the invaginating cells obUterate the blastula cavity 

 and come in contact with the outer layer of cells (Fig. 19). The new cavity thus 

 formed is the primitive gut or arch enteron and its narrowed mouth is the blastopore. 

 The outer layer of cells is the ectoderm, the inner, newly formed layer is the ento- 

 derm. The entodermal cells are henceforth concerned in the nutrition and metab- 

 olism of the body. The embryo is now termed a Gastrula (little stomach). 



Reptiles and Birds. — The germinal disc, or blastoderm, in these animals lies 

 Uke a cap on the surface of inert yolk (Fig. 3). Since the enormous amount of 

 yolk makes gastrulation as in Amphioxus and amphibians impossible, the process 

 exhibits marked modifications. 



