68 ADAM SEDGWICK. 



Wolffian blastema applies {oide fig. 1); but behind this 

 region the development is different. 



Here, even before the segments are formed, it is found 

 that the cell mass, which on segmentation gives rise to the 

 intermediate cell mass, is distinctly separate from the thick 

 epithelium of the body-cavity, but attached to the cell mass, 

 which will give rise to the protovertebra (fig. 6, taken 

 from a chick with twenty-six protovertebrae behind the last 

 protovertebra) . 



And this separation is apparently retained through later 

 development. The cell mass (*cm', fig. 6), in the next stage 

 (fig. 7, taken from twenty-ninth segment of a chick with 

 twenty-nine protovertebrae) is obviously the Wolffian blas- 

 tema which, in a still later stage (fig. 10, from the twenty- 

 ninth segment of a chick with thirty-four), gives rise to the 

 commencing Wolffian tubule. 



The same fact may be seen by comparing figs. 8 and 9, 

 fig. 8 being taken from the twenty-fourth segment of a chick 

 with twenty-six, and fig. 9 from the twenty-fourth segment 

 of a chick with twenty-nine. 



It will be observed, by inspection of figs. 6, 7, 8^ that the 

 peritoneal epithelium which adjoins the Wolffian blastema is 

 thick, as it is elsewhere; while later (figs. 9, 10) it is in 

 the same spot thin, as it is, or will be, in most other parts 

 of the body-cavity. 



No doubt this thick epithelium does, in the process of 

 becoming thin, bud off cells, which travel inwards, and some 

 of which may help to form the definite Wolffian blastema 

 (figs. 9, 10). But this process takes place everywhere. In 

 fact, at an early stage of development almost all the meso- 

 blast cells are represented by the thick lining of the body- 

 cavity ; and it is by a process of growth inwards of cells from 

 this that most of the connective tissue, &c., of the wall of the 

 body and gut is derived ; and, therefore, from the analogy of 

 the fate of the cells growing out from the body-cavity wall 

 in other places, we might fairly assume that those growing 

 out from that particular part adjoining the intermediate cell 

 mass or Wolffian blastema give rise, not to the Wolffian 

 tubules, but to the connective tissue and blood-vessels of the 

 Wolffian body. This is rendered highly probable from a 

 consideration of some observations I have made on the de- 

 velopment of the segmental tubes in Elasmobranchs, which 

 point to the conclusion that the epithelial lining is derived 

 from the cells of the intermediate cell mass. However that 

 may be, of one fact there can be no doubt, viz. the cells from 

 which the Wolffian tubules develop are not derived from 



