50 WILLIAM MORTON WHEELER, 



Althougli the reproductive organ originally extends some distance 

 back of the cloacal termination of the pronephric duct, the subsequent 

 growth of the posterior part of the body takes place in such a way 

 that the posterior end of the mass of sex-cells lags behind and 

 ultimately terminates a short distance in front of the posterior ends 

 of the ducts. 



In regard to the development of the posterior terminations of the 

 pronephric ducts and the abdominal pores, I have little to add to the 

 observations of previous writers. In Ammocœtes 8 — 9 mm long the 

 ducts still open into the hind part of the intestine where it bends 

 down to open on the ventral surface of the body. This common 

 receptacle for the intestine and pronephric ducts may be called the 

 cloaca. The body cavity becomes somewhat tubular as it runs back 

 on either side to terminate blindly against the cloacal wall behind and 

 below the openings of the pronephric ducts. Even in these very young 

 Ammocœtes the peritoneal epithelium at these two points is very much 

 thickened and somewhat modified, indicating the places where the 

 abdominal pores will finally appear. Beyond this condition, which 

 persists throughout the whole of larval life, as several investigators 

 have shown, I have not followed the development. For the final stages 

 in the formation of the abdominal pores, the urinogenital papilla etc., 

 the reader is referred to Bujor's account (1891). 



It will be seen that my account of the early development of the 

 reproductive organ in the lamprey agrees in the main with Goette's 

 (1891). In one matter, however, I am inclined to take exception to 

 his remarks. He claims (p. 53) that the primitive reproductive cells 

 are "ursprüngliche Mesodermelemente und nicht etwa vom Darmblatt 

 her eingewanderte Zellen". Goette did not see the sex-cells earlier 

 than his stage 7 (my stage 4), although they are present as early as 

 my stage 2 (Fig. 34 gon). In Goette's stage the large yolk-laden 

 sex-cells may certainly be described as lying in the mesoderm , but 

 this can hardly be said of the cells in stage 2, e. g. on the right of 

 Fig. 34. Here I should say that the mesoderm is not yet differenced 

 from the entoderm, and that the sex-cells must be assigned to the 

 entoderm till the middle germ-layer is recognizable as such. 



