THE DEVELOPMENT OF PALLTDINA VIVIPARA. 109 



well-developed velum, and already a slight swelling ventrally, 

 Avliicli is the rudiment of the foot. The chief points in the 

 anatomy are shown in fig. 10, which is a sagittal section 

 through an embryo of this stage. The gut {st.) is a simple 

 sac opening posteriorly by the anus (a.), and ending blindly 

 anteriorly where it abuts against an insinking of the 

 ectoderm (m.), the rudiment of the stomodaeura. No very 

 clear differentiation of parts is yet visible in the gut, but the 

 ventral wall begins to show the vacuolated structure charac- 

 teristic of the liver at a later stage. Dorsally to the stomo- 

 dteal invagination the velum is seen cut twice (v.), and more 

 posteriorly is seen the shell gland (s.g.), a deep sac, widely 

 open to the exterior in other sections of the series. The 

 mesoderm at this stage is represented simply by scattered 

 cells. 



Stage B (fig. B), shows considerable advance upon the 

 last. The foot (/.) has grown out to form a prominent pro- 

 jection on the ventral surface. The shell gland is partially 

 evaginated, and begins to form the visceral hump {v. h.), 

 which, however, is still partly surrounded by a groove, 

 deepest behind, and gradually disappearing anteriorly. The 

 velar area has increased in size, and the tentacles (t.) are dis- 

 appearing. The stomodeeum has now broken through into 

 the archenteron, and considerable differentiation has taken 

 place in the latter. The middle portion has swollen and 

 forms the stomach (st.), which lies at the apex of the visceral 

 hump {v. h.), and from which the rectum runs downwards and 

 backwards to open in the middle line behind (a.) The liver 

 [l.) is an oval structure, sloping downwards and forwards 

 from the apex of the visceral hump, where it communicates 

 mostly with the stomach. The opening into the stomach is 

 still so wide, and the demarcation between the two organs so 

 vague in this region, that it is difficult to determine their 

 exact relations, but the liver appears to lie to the left, and 

 ventrally behind, while in ventral views of the whole embryo 

 it can clearly be seen to pass below the oesophagus and to the 

 right side anteriorly. It seems then to be an outgrowth of 



