268 EMBBYOLOGY OF THE LOWEK VEETEBEATES oh. 



As development goes on the genital ridge increases in depth and 

 is now termed the genital fold. This is composed of peritoneal 

 epithelium covering a supporting and, later on, vascular core of mesen- 

 chymatous connective tissue. 



The rudiment of the actual gonad in the strict sense consists of a 

 thickening of the peritoneal epithelium covering the genital fold — 

 the germinal epithelium. This thickened germinal epithelium may 

 extend over both mesial and lateral surfaces of the genital fold as in 

 most Amphibians, Eeptiles and Birds or it may be restricted to its 

 lateral (Ichthyophis ? , most Teleosts) or median (Elasmobranchs 

 except in very early stages, Ichthyophis 3 ) surface. 



Of the more primitive holoblastic Vertebrates the Amphibia are 

 the only group on which detailed observations on the origin of the 

 gonad have been recorded. We shall accordingly summarize the 

 early stages in the development of ovary and testis in this group and 

 where possible in its more primitive subdivision the Urodela. 



Fig. 140 illustrates the earliest stages of the gonad so far identified 

 in Urodeles, as described by Schapitz (1912) for the Axolotl. Fig. A 

 is taken from an embryo in which the protovertebral stalk or nephro- 

 tome is not yet completely restricted off from the myotome. On its 

 outer side is seen the rudiment of the archinephric duct. The stalk 

 is continuous ventrally with the lateral or splanchnocoelic mesoderm. 

 In its inner portion certain of its cells (e.g. the two adjoining cells in 

 the figure in which the nucleus is shown in a darker tone) are begin- 

 ning to show recognizable indications of nuclear and cytoplasmic 

 features which are characteristic of the gonad later on. It will be 

 borne in mind that the wall of the protovertebral stalk is morpho- 

 logically part of the coelomic wall to which therefore these gonad 

 cells also belong. In the sections shown in B and the mass of 

 cells showing these peculiarities has become more and more distinctly 

 marked off from the lateral mesoderm (mes) and may now be spoken 

 of definitely as the gonad. In the stage illustrated by D the lateral 

 mesoderm is seen to be spreading inwards towards the mesial plane 

 ventral to the gonad and it is beginning to show here and there a 

 distinct split separating its somatic and splanchnic layers. In the 

 later stages (E, F, G) this split becomes a patent cavity — the splanch- 

 nocoele (splc) — and the gonad is seen to lie on the dorso-lateral side 

 of this, separated from the actual cavity by the somatic layer of 

 peritoneal epithelium. In the last stage figured (G) the gonad is 

 causing a slight bulging of the peritoneal lining into the splanchno- 

 coele : this bulging is the incipient genital ridge (g.r). 



During the earlier of the stages illustrated the gonocytes gradu- 

 ally acquire the superficial histological characters of germ-cells. The 

 cell-body is larger than that of the other cells, it remains full of yolk 

 particles, and in the spaces between the latter are to be seen fine 

 granules of dark pigment. The nucleus is elongated or lobed in 

 shape, the chromatin distributed in fine particles so that the nucleus 

 as a whole stains less deeply than do the nuclei of other cells, and 



