THE INTESTINAL GERM- VESICLE. 29 1 



of many Invertebrate Animals {e.g. of the Monoxenia, Fig. 

 22, F, G). This true germ-membrane vesicle has, up to the 

 present time, been universally regarded as homologous with 

 the germ-vesicle of Mammals. It is not so, however. The 

 so-called " germ-vesicle " of Mammals and the true germ- 

 membrane vesicle of the Amphioxus and of many Inverte- 

 brates are entirely different germ-forms. The latter (the 

 Blastula) precedes gastrulation. The former {vesicula 

 hlastodermica) follows gastrulation. The globular wall of 

 the blastula is a true germ-membrane (Blastoder7na), and 

 consists entirely of cells of one sort (blastoderm-cells) ; it 

 is not yet specialized into the two primary germ-layers. 

 On the other hand, the globular wall of the mammalian 

 " germ- vesicle " is the specialized skin-layer (exoderma), 

 and a circular disc of entirely different cells lies at a point 

 on the inside of this ; this disc is the intestinal layer 

 {entoderiiia). The spherical cavity, filled with clear liquid, 

 in the interior of the blastula, is the cleavage-cavity. On 

 the other hand, the similar cavity in the interior of the 

 mammalian germ-vesicle is the yelk-sac cavity, which is 

 joined on to the developing intestinal cavity. 



On all these grounds, which have been very recently 

 brought to light by the researches of Van Beneden, it is 

 very necessary to recognize the secondary " intestinal germ- 

 vesicle " of Mammals (Gastrocystis) as a peculiar germ-form, 

 occurring only in this class of animals, and as quite distinct 

 from the " germ-membrane vesicle " (Blastula) of the Am- 

 phioxus and of the Invertebrates. The wall of this mam- 

 maUan " intestinal germ-vesicle " consists of two distinct 

 parts. Far the larger part is one-layered, and is formed by 

 the exoderm. For the smaller part, the cii'cular disc, which 



