20 Mr. R. C. Punuett. On the Composition and Variation* 



Iii the younger stages, A 0, it is net possible to determine the sex 

 as the mixipterygium is not yet distinguishable. The table brings out 

 the following facts : 



(1.) The number of nerves forming the nervus collector is greater 

 than in older embryos or adults. This has also been shown by 

 Braus for Spinax niger (11), p. 620, and by myself for Mustehis 

 lcevis((12\ p. 347). 



(2.) The number of post-girdle nerves is greater in younger embryos 

 than in older embryos or adults (cf. Table III). 



(3.) The last two or three post-girdle nerves form a posterior collector 

 (cf. (12), p. 347, and (11), p. 566), a structure which is quite 

 separate from the " Langsstamme " occurring in the adults. 

 These latter structures also are found in embryos of 42 mm. 

 and upwards. In the two younger embryos (A and B) they 

 are not yet fully formed (cf. fig. 7). I have only observed a 

 posterior collector in one case among older embryos and never 

 in adults. In the older embryo in which it occurred there 

 were eleven post-girdle nerves, and the two branches of the 

 posterior collector were extremely fine. 



There is a certain amount of variation in the number of the 

 pelvic foramina. Asymmetry occurred in one case, C. 



The fact that the fin innervation area has a greater extent both 

 caudally and rostrally in the embryo than in the adult, offers no criterion 

 between the rival theories. On the side fold excalation theory we 



(4.) 



