48 WILLIAM MORTON WHEELER, 



portal in the spiral valve on the mid-dorsal, the subintestinal on the 

 mid-ventral surface of the gut. 



In the Aramocœtes 7 cm long above described the completed 

 larval condition of the mesenteric vessels at the i)osterior end of the 

 intestine is not yet developed. Schneider found four veins connecting 

 the intestine with the subcardinal sinuses and each of these was ac- 

 companied by an arteriole from the aorta to the intestine. These 

 last had not yet developed in the young Ammocœtes which I studied, 

 and only a single vein was present leading from the intestine to the 

 caudal vein. Miss Claypole (1894, p. 131) has studied this region 

 in P. marinus dorsatus but gives neither the exact number nor 

 character of these vessels. She says: "Near the caudal end some 

 dorsal bands are formed much larger than those of the larva and 

 varying in number from three to six." 



The foregoing remarks show that there is still something to be 

 learned concerning the relations of the vessels of the mesonephros to 

 those of the intestine. That there is an intimate relation admits of 

 no doubt but its exact nature remains to be elucidated. The relations 

 established by Bert and Miss Claypole between the subcardinal 

 sinus and the portal vein of the adult are certainly interesting in 

 connection with facts recently brought to light by three Italian in- 

 vestigators, Bizzozero, Giglio-Tos and Ascoli. Bizzozero (1892) 

 and GiGLio-Tos (1897) find that leucocytes aie formed in the lymphoid 

 tissue of the spiral valve of the intestine, and Ascoli (1898) claims 

 that they are also formed by mitosis of preexisting leucocytes in the 

 lymphoid tissue of the nephric lobes beneath the subcardinal sinuses. 

 Hence both the portal and subcardinal vessels drain leucocyte- 

 forming tissue and ultimately pour their contents into the liver 

 capillaries. 



3. The Deyelopment of the Reproductive Organs. 



The reproductive organs can be recognized in the posterior 

 portion of the embryo as early as stage 2. This region is represented 

 in cross-section in Fig. 34. The mesoderm is still confluent with the 

 entoderm laterally and only just developing into the myotomes mesially. 

 There is as yet no trace of the pronephric duct in this region, but 

 just lateral to the myotome a few very large rounded masses of yolk 

 (gon) are seen, each containing a nucleus and more or less distinctly 

 marked oft" from the adjacent entoderm elements. These large masses 

 are the primitive reproductive, or sex-cells. They can hardly be 



