8 TV. p. E. WELDON. 



the convexity being directed outwards ; this condition is 

 shown for vertebral regions in fig. 17, for intervertebral in 

 fig. 18. 



At a stage with eleven protovertebrae, the vertebral portions 

 of the intermediate cell mass, behind the fourth protovertebra, 

 acquire a circular lumen, which is bounded by a single layer 

 of columnar cells ; this condition is seen in fig. 19. In fig. 20, 

 which represents a section passing through the end of the 

 same protovertebra as that from which fig. 19 is taken, the 

 lumen is smaller; in the intervertebral region behind the 

 lumen altogether vanishes, and the solid, swollen cell mass pre- 

 sents an appearance exactly like that seen in the preceding 

 stage (fig. 18). 



There is thus formed a series of cavities in the continuous 

 intermediate cell mass, each situated opposite a protovertebra, 

 and having its walls continuous both vvith the protovertebra and 

 with the peritoneal epithelium. These cavities arc separated 

 from one another by the solid intervertebral parts of the inter- 

 mediate cell mass. 



In embryos with eleven protovertebrae there are five of these 

 vesicles, opposite the fifth to the tenth protovertebra, the last 

 two somites being as yet without them. In these last somites 

 the intermediate cell mass is swollen and solid, as in the 

 anterior region of an earlier embryo. 



These cavities are, as will be seen from their subsequent 

 history, the segmental vesicles described by Rathke and subse- 

 quent writers. 



They have hitherto been described entirely separate from 

 one another, and have been supposed (Braun., loc. cit.) to arise 

 as invaginations of the peritoneal epithelium. 



When twelve protovertebrae are present the Wolffian duct 

 begins to appear as a solid cord of cells, splitting off in the 

 intervertebral region only from the intermediate cell mass, 

 and passing, in the region of each protovertebra, into the wall 

 of a segmental vesicle. 



Figs. 21 — 23 represent three sections through about the 

 sixth and seventh somites of an embryo with twelve proto- 



