1V GONAD 267 



which mark it off from all other Vertebrates. Bearing in mind 

 however that the general arrangement of the mesoderm of the adult 

 Amphioxus, which also shows striking peculiarities, is preceded by a 

 condition in ontogeny which there is reason to regard as more nearly 

 primitive than occurs in any other Vertebrate— the possibility at 

 once suggests itself that this may also be the case with the gonad. 

 Consequently it becomes important to enquire what are the early 

 conditions of the gonad in Amphioxus and whether it is reasonable 

 to interpret the conditions in the more typical vertebrates as being 

 modifications of those illustrated by Amphioxus. 



The earliest so far recognized stage of the gonad (Boveri, 1892 ; 

 Zarnik, 1904) consists of a thickened portion of coelomic epithelium 

 at the ventral end of the mesoderm segment i.e. in the region where 

 at an earlier stage the segmented part of the mesoderm was continuous 

 with the portion which loses its segmentation. The thickening lies 

 close to the headward boundary of the segment and within its ventral 

 angle. As the segment has already become nipped off from the 

 lateral mesoderm it is not possible to say from actual observation 

 that the thickening belongs to the splanchnic rather than the 

 somatic wall though this is probable from the condition in the more 

 typical vertebrates. The genital thickening is repeated over a 

 number of segments (from about the 9th or 10th to about the 34th 

 or 35th — 'Zarnik). 



There are then three important points to be gathered from the 

 study of the origin of the gonad in Amphioxus : — 



(1) It arises as a thickening of coelomic epithelium i.e. it shows 

 the mode of origin characteristic of coelomate animals in general, 



(2) It arises close to the boundary of segmented and unsegmented 

 mesoderm, and 



(3) It arises on the dorsal side of that boundary. 



In the more typical Vertebrates the ovary or testis first becomes 

 clearly recognizable as a rule in the form of a longitudinal ridge — 

 the genital ridge — which runs along the dorsal wall of the splanchno- 

 coele on each side, at a varying distance from the line of attachment 

 of the dorsal mesentery, and projects into the splanchnocoelic cavity. 

 The genital ridge commonly extends over a greater antero-posterior 

 extent than does the functional gonad later on — e.g. in the Salmon 

 of the 185th day it extends from about the level of the fourth trunk 

 myotome back to behind the anus (Felix). The restricted portion of 

 the ridge which is destined to develop into functional ovary or testis 

 is termed by Felix the gonal portion to distinguish it from the 

 portions in front (progonal) and behind (epigonal) which remain 

 sterile. 



The relatively great anteroposterior extent of the gonad during 

 early stages in its development is probably to be regarded, along with 

 the greatly elongated condition in the adult of some of the more archaic 

 Vertebrates, as evidence that at one period of evolution the gonad 

 extended throughout the whole length of the splanchnocoele. 



