GENERAL EMBRYOLOGY 1613 



dermic vesicle, where they are in intimate contact with the envelop- 

 ing layer (this part of the enveloping layer being, as stated, Rauber's 

 covering layer). Elsewhere, the inner cell-mass is separated from 

 the enveloping layer by the segmentation-cavity and its fluid. 



The enveloping layer constitutes the blastoderm, which, at this 

 stage, is unilaminar. 



The cells of the inner cell-mass, which lie next the segmentation- 

 cavitv, now begin to spread out, and they ultimately form a layer 

 which lines the enveloping layer. This lamina represents the 

 primitive entoderm. The blastoderm is now bilaminar, its outer, 

 or enveloping, layer representing the primitive ectoderm, whilst 

 its inner layer corresponds to the primitive entoderm. 



Prior to the completion of the entodermic layer, certain changes 

 take place within the remainder of the inner cell-mass. The lower 

 cells form a thickened group, known as the embryonic bud, or disc, 

 from which the embryo is devel- 

 oped. Within the upper cells inner C*lI.MaM^^^^G«nninal Layer 



vacuoles appear, which rapidly 

 increase and coalesce so as to form 

 a cavity within the upper part of 

 the inner cell-mass, which is called 

 the amniotic cavity. The roof of 

 this cavity is formed directly by 

 the superficial cells of the inner 

 cell - mass. Superficial to this 

 lamina, at first, there is that part 

 of the enveloping layer, or extra- 

 embryonic ectoderm, which is 

 known as Rauber's laj-er. The floor _ 



of the amniotic cavity is fomied p,^ eej-^S^TI^MEXTATioN 

 by a layer of cells, derived from c.wity of Blastodermic 



those which form the amniotic sur- Vesicle. 



face of the embryonic bud, the 



cells of that bud being derived from the inner cell-mass. This floor- 

 layer of cells and the direct roof-layer of cells, both of which are 

 derived from the inner cell-mass, constitute the embryonic ectoderm, 

 as distinguished from the extra-embryonic ectoderm, wJiich is repre- 

 sented by the enveloping layer. The embryonic ectoderm, which 

 surroim(6 the amniotic cavity, becomes continuous with the extra- 

 embryonic ectoderm, or enveloping layer, at the periphery of the 

 roof of the amniotic cavity; and that part of the enveloping layer 

 which enters into the roof of the cavity (Rauber's laver) disappears. 

 The upper pole of the blastodermic vesicle is now covered by 

 embryonic ectoderm. 



GastrolatioD. — In Amphioxus the blastomeres or segmentation-cells of the 

 fertilized ovum are disposed at an early period in one layer, and there is no 

 morula-stage. This single layer of cells constitutes the unilaminar blasto- 

 derm, which encloses a cavity, and the ovum is now a blastula or blasto- 

 dermic vesicle. 



