140 W. F. R. WELDOxV. 



According to him, the first part of the urino-genital system 

 which appears is the Wolffian duct. He says : " In an em- 

 bryo of Lacerta agilis, barely 5 mm. long, in sections just 

 below the heart, I find the Wolffian duct lying close to the lateral 

 mesoblast plates, in a region belonging neither to these nor to 

 the protovertebrae, but lying between the two as a semicircular 

 mass of cells, sharply defined towards the ectoderm, but 

 passing gradually into the lateral mesoblast; in the middle of 

 this cell mass is a lumen . . . . " which he considers 

 to be the lumen of the Wolffian duct. 



In the next stage described by Braun, a number of seg- 

 mentally arranged vesicles are present, which are for a short 

 time attached to the peritoneal epithelium, their cavities also 

 opening for a short time into the body cavity, but which 

 afterwards break away, form the well-known S-shaped tubes, 

 and communicate with the Wolffian duct. 



From this account it is evident that Braun has not investi- 

 gated embryos less than 5 mm. long. I have been fortunate 

 enough to obtain younger embryos, and have been led to some- 

 what different conclusions. 



On the formation of the protovertebrae, each protovertebra 

 does not at once become completely separated from the lateral 

 mesoblast, but remains connected at a certain point with a 

 continuous solid ridge of tissue, generally in early stages 

 about two cells thick, which projects inwards from the peri- 

 toneal epithelium, thus forming an " intermediate cell mass " 

 comparable with the structure so called in birds. 



Figs. 15 and 16 show the characters of this ridge in an 

 embryo of about seven protovertebrae ; fig. 15 is taken from 

 a vertebral region, and shows the ridge {i. c. m.) connecting 

 the protovertebra with the peritoneal epithelium ; fig. 16 is 

 from the next intervertebral region, showing the ridge pro- 

 jecting freely inwards from the peritoneum. In fig. 16 traces 

 of a prolongation of the body cavity into the intermediate 

 cell mass may be observed. In an embryo with ten pro- 

 tovertebrae this cell mass, without losing its connection with 

 the protovertebrae^ swells up and becomes semicircular in 



