3^4 THE EYOLUnON OF MAK. 



distinctioii between the exocoelar, or the parietal corfom- 

 epithelium, which originates from the skin-fibrous layer, and 

 the endocoelar, or the visceral ccelom-epithelium, which 

 ie derived fnym the intestinal-fibrous layer. The former 

 is, perhaps, connected with the male germ-epithelium (the 

 rudiment of the testes), the latter with the female germ- 

 epithelium (the rudiment of the ovary). (Cf. Chapter XXV.) 



TABLE XL. 



ST*nM4nC SUSTIT OV THE MOST IMPOKTANT PesIODS IN THS PhTLOOSJIT 



07 THE Human Yasculae System. 



I. Fint Period : Vascular System of the earlier Seolecida. 

 Between the skin-covering and the intestinal wall is formed a simple 

 bodj-oavity (cesloma), or a perienteric cavitj (as in the extant Bryo»oa and 

 otber Coslomati). 



IL Second Period : Veueular System of the wtore recent Seolecida. 



The first real blood-ressels form in the intestinal wall (in the intestinal* 

 Abroos layer), a dorsal vessel in the central line of the dorsal side of the 

 intestinal tnbe, and a ventral vessel in the central line of its ventral side. 

 The two vessels are oonnected bj several oiroolar vessels, enoiroling the 

 inteatixM. 



nL Third Period : VmeeuUtr System cf ike ea/rUer Chordoma. 



Bj the modification of the anterior half of the intestine into a gill- 

 intestine, the anterior section of the ventral vessel becomes a gill-arterj, 

 and the anterior section of the dorsal vessel a gill- vein | between the two 

 a gill capillary network develops. 



rf. FovHh Period : VaseuJair System of the mare reeem t Chordoma, 



l%e portion of the ventral vessel, Ijing immediately behind the gfB>- 

 te a simple beart-poook (Aaddian). 



