22 INTRODUCTION. 



adjacent parts of the yolk, at the close of segmentation. Seg- 

 mentation, when confined to part of the egg, is spoken of as 

 meroblastic ; and when, as in the hen's egg, it is limited to a 

 circular patch on the surface of the egg it is further distin- 

 guished as discoidal. 



Another type of meroblastic segmentation is presented by 

 the centrolecithal eggs of Arthropods. Here, there is no localised 

 germinal disc, but the whole surface of the egg consists of a 

 layer of protoplasm free from yolk-granules, in which segmenta- 

 tion occurs almost simultaneously at all parts ; such a mode 

 of segmentation may be distinguished as superficial. 



The principal types of segmentation, described above, may 

 be tabulated as follows : — 



I. Holoblastic or complete segmentation. 



A. Equal : as in the alecithal egg of Amphioxus. 



B. Unequal : as in the telolecithal egg of the frog. 

 II. Meroblastic or partial segmentation. 



c. Discoidal : as in the telolecithal egg of the chick. 



D. Superficial : as in the centrolecithal eggs of Arthropods. 



The Germinal Layers. 



At the close of segmentation the whole of the egg, or, in 

 cases of meroblastic segmentation, a part only of it, is divided up 

 into cells or blastomeres. These blastomeres very early become 

 arranged in two layers ; an outer layer, the epiblast, which covers 

 the surface of the embryo ; and an inner layer, the hypoblast, 

 which lines a cavity within its interior. Epiblast and hypoblast 

 form the two primary germinal layers of the embryo : the epi- 

 blast becomes ultimately the epidermis or outer layer of the 

 skin ; while the hypoblast becomes the epithelial lining of the 

 alimentary canal ; the cavity surrounded by the hypoblast, 

 spoken of as the archenteron, forming the first commencement 

 of the digestive tract. Figs. 8 and 9 represent early larvse of 

 Amphioxus which have reached the stage described. 



The details of development of epiblast and hypoblast, and 

 more especially the mode of appearance of the archenteric cavity, 

 are subject to great modifications in different groups of animals, 

 but the essential relations are in all cases as described above. 



Between epiblast and hypoblast a third layer of cells, the 

 mesoblast, appears at a later stage, usually derived, directly or 



