LAR VAL DE VEL 0PM ENT. 



155 



the primitive sexual cells 

 make their first appearance 

 in the segmented area of 

 the trunk at the base of 

 the somites. Later on, by 

 differential growth, they 

 come to lie on the dorsal 

 wall of the unsegmented 

 peritoneal cavity, and their 

 primitive segmental origin 

 is entirely obscured ; while 

 in Amphioxus the primitive 

 segmentation of the gonads 

 is maintained throughout 

 life. 



This forms another most 

 interesting example of the 

 way in which the adult 

 Amphioxus, in the details 

 of its organisation, essen- 

 tially resembles the em- 

 bryos of the higher types. 



Fig. 90. Horizontal section through 

 the ventral portion of six consecutive 

 mesodermic somites of an embryo of 

 Pristiurus, to show the segmental origin 

 of the sexual elements. (After RUCKERT). 



c. Cavities of somites, g.c. Sexual 

 cells. 



This observation of Riickert's has 

 recently been doubted, with how much 

 justice it is difficult to say, by MlNOT 

 (Gegen das Gonotom. Anat. Anz. IX. 

 1894. pp. 210-213). 



GENERAL CONSIDERATIONS. 



We will now pass on to give a general interpretation of 

 some of the principal phenomena which are presented to 

 us in the development of Amphioxus. 



Larral Asymmetry. 



By far the most prominent feature of the fully formed 

 larva is its astounding asymmetry, and it is extremely 

 important, from a morphological point of view, to form a 

 just conception of it. 



