214 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



to loss of yolk and consequent modification of processes. From 

 almost the first the segmentation is irregular ; and there results, 

 exactly how is not known, a solid sphere consisting of an outer 

 layer of hyaline cells surrounding a mass of more granular cells, 

 one of which reaches to the exterior through a gap, closed 

 later, between the cells of the outer layer. Now the solid mass 



expands into a hollow sphere, the 

 blastodermic vesicle, the outer 

 layer of cells becoming greatly 

 flattened, the inner adhering to 

 one side of them in a small len- 

 ticular mass, the cavity of the 

 vesicle being filled with fluid. 

 The lenticular mass becomes 

 three layered, increases in extent, 

 and gradually extends around 

 under the outer layer so that the 

 whole vesicle is eventually two 

 layered throughout. The cen- 

 tral portion of the lenticular 

 mass remains thicker than the rest, and in this place the em- 

 bryo arises, a primitive streak being formed, but nothing that 

 can with certainty be called a blastopore appears. 



So far there is little dispute as to the facts, but as to their 

 interpretation the views are various, some regarding the outer 

 layer as ectoderm, others as entoderm ; while the inner cell mass 

 is regarded by some as purely entodermal, by others as giving 

 rise to both ectoderm and entoderm. For details reference 

 must be made to special works on vertebrate embryology. 



FlG. 220. Diagram of mammalian 

 blastodermic vesicle. *', inner cell 

 mass. 



