130 COMPARATIVE ANATOMY. 



terior or dorsal surface. A portion of the vitellus becomes 

 surrounded by the large median layer of tissue. The re- 

 maining unappropriated area is defined externally by a mem- 

 brane derived from the inner layer of the blastodermic mem- 

 brane, and communicates with the oesophagus through a 

 narrow canal. It resembles in form the umbilical vesicle of 

 vertebrates, from which, however, it markedly differs in ab- 

 sence of connection with the abdomen. The embryo is more 

 fully developed before it is made independent of its capsule 

 than is the case in other Invertebrata. 



ARTICULATA. The following changes are observed subse- 

 quent to impregnation in the egg of an articulate animal. 

 After segmentation, which may be either complete or par- 

 tial, a clear albuminoid space (germinative area) is found 

 toward the ventral surface of egg, and the yolk, or vitelline 

 area, upon the dorsal. The first trace of organization is seen 

 in the formation of the germinal layer (blastodermic mem- 

 brane), which in time surrounds the yolk, its thickened 

 portion of the area within the germinative area being called 

 the germinal disk. This layer divides into two ; the outer (ani- 

 mal, serous or nervous) layer is devoted to the development 

 of the skeleton and nervous system, the inner (vegetative, 

 mucous or subsidiary) layer is given to the evolution of the 

 organs of nutrition. This disk soon becomes transversely 

 wrinkled, indicating the points of formation of the future seg- 

 ments. From the ventral sides of the disk two folds descend 

 to form a groove, within which is developed the ganglionic 

 chain of nerves. 



Entozoa. See p. 161. 



Annelida. Segmentation complete. Eggs spherical and 

 ciliated. Two types with Hirudinei. After the vitellus has 

 divided into many large cells, a central one becomes distin- 

 guished from the others by its still further division. This 

 becomes the digestive tube. The others, still dividing, form 

 a primitive embryonic part, in which appear the future ven- 

 tral and nervous portions. With the Branchiata there is a 

 complete metamorphosis. The segmentation of the vitellus 

 is uniform throughout, and this last is finally changed into a 



