270 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



the mesoderm segments. The discontinuity becomes less marked in 

 the later stages but even in an 18-day embryo Schapitz found the 

 gonad still consisting on one side of the body of metamerically 

 arranged blocks while on the other it had become a continuous 

 strand, except for a single small isolated piece posteriorly. 



From what has been said it seems clear that the gonad of the 

 Urodela is a derivative of the coelomic wall lying close to the 

 boundary between the segmented and the unsegmented (lateral) 

 portions of the mesoderm. As in early stages it consists of blocks 

 with a roughly segmental arrangement it would appear to lie on the 

 dorsal or nephrotomal side of the boundary mentioned. There is no 

 apparent reason for declining to interpret this early segmented stage 

 of the gonad as a persistent trace of a primitive segmental arrange- 

 ment like that of A?nphioxus. 



The tendency for the segmented condition to disappear in the 

 typical Vertebrates is adequately explained by the gradual dorsal- 

 ward encroachment of the unsegmented splanchnocoele. The 

 boundary between segmented and unsegmented (lateral) mesoderm 

 has altered much in position during the course of evolution, and 

 there is no adequate reason to suppose that this boundary is not still 

 a fluctuating one and if it is so we may expect varying traces of the 

 original segmented condition to present themselves during develop- 

 ment. 



The gonad has been described as being paired throughout but it 

 may be mentioned that various observers have noticed an unpaired 

 condition at one or other period during the early stages of develop- 

 ment. This appears to be adequately interpreted as a secondary 

 fusion similar to that occurring between the right and left opistho- 

 nephros in a Teleost or in Protopterus rather than as a primary 

 condition. 



We have traced back the gonad to its first recognized beginnings 

 in one of the relatively primitive holoblastic Vertebrates. Before 

 passing on to its farther development it has to be noticed that there 

 exists a considerable volume of evidence pointing to the existence of 

 additional germ - cells which arise independently of the . coelomic 

 lining and some of which migrate into the germinal epithelium and 

 may give rise eventually to functional gametes. It is not proposed 

 to describe this evidence as it has not as yet, in the present writer's 

 opinion, reached the stage of being convincing. It does not appear 

 to have been satisfactorily demonstrated that the supposed extra 

 gonocytes are really gonocytes at all rather than somatic cells. 

 What is needed to provide such a demonstration is a careful study 

 by skilled cytologists of the nuclear features of these cells, to 

 determine whether there are any definite nuclear characters (such as 

 Boveri discovered to be present in the gonocytes of Ascaris megalo- 

 cephala) showing them to be beyond doubt gonocytes and affording & 

 means of tracking them down in their supposed migration. Mere 

 shape and staining capacity of the nucleu's as a whole, or presence of 



