46 



General Part. 



The unsegmented portion corresponds to that part in the forms 

 mentioned under (4), which does segment, but which unites later, and 

 is invaginated with some of the adjacent cells into the remaining cell- 

 mass, the process taking place as in the way last mentioned (Pigs. 

 ■34, 35) . An enormous mass of unsegmented substance, the food 



Fig. 35. Diagrammatic representation of gastrula-formation in Vertebrata, with 

 partial segmentation (Selachians, Teleostians, Eeptilia, Aves) ; compare preceding figures ; 

 the letters are the same as in these, with the exception of en' the unsegmented part of the 

 hypoblast (the yolk) ; en the segmented hypoblast. It must be noticed that the yolk is 

 usually, relatively much greater than is figured here. — Orig. 



yolk, becomes enclosed by the archenteron; is gradually absorbed 

 by the cells; and thus serves as nutriment during development. 

 These animals are, moreover, peculiar, in that invagination proceeds 

 very slowly. An advanced stage of development is reached by the 

 time that gastrulation is complete. No notice has been taken of this 

 in the figures. 



6. In most of the Arthropoda, as in the animals just referred to, 

 partial segmentation obtains, but the food yolk lies centrally ; 

 surrounded by the segmentation spheres. The segmentation cavity is 



