LATER DEVELOPMENT OF CHICK AND RABBIT 425 



involuntary muscles are derived from the splanchnic mesoderm. 

 To return now to the more ventral mass of the somite, we find that 

 its cells proliferate actively and take up a position near the noto- 

 chord, and it is then distinguished as the sclerotome. This spreads 

 around the notochord, the neural canal and the dorsal aorta, and its 

 cells secrete an intercellular substance. Some time afterwards the 

 sclerotomes acquire a secondary segmentation, and each becomes 

 divided transversely in the middle of the somite. The posterior 

 half of each one unites with the anterior half of the next succeeding, 

 and the block? of tissue formed in this way of course come to alternate 

 with the original somites. The sclerotome tissue gives rise to the 

 skeletal elements of the axial skeleton save for a large part of the 

 skull. 



The intermediate cell mass is also termed the nephrotome, in 

 order to indicate its close relation with the excretory system, and 

 in it are developed the constituents of the kidneys. In all higher 

 Vertebrata three successive excretory organs make their appearance, 

 and are known as the pro-, meso- and meta- nephros respectively. 



The first of these, the pronephros, is never functional in the chick 

 at any time, and is represented only by degenerate remnants. Its 

 vestigial tubules may appear in the fifth to the sixteenth somites, 

 but are only marked in the last six or so, and there is a certain 

 amount of variation in their number and arrangement. The first 

 show in this region in the middle of the second day in the form of 

 small solid cellular buds on the postero-dorsal surface of the nephro- 

 tome. These buds elongate, and their distal extremities bend over 

 and unite with the corresponding portion of tubules behind, so that 

 they come to form a continuous rod of cells, which represents the 

 Wolffian duct. Although we term these structures the tubules, it is 

 somewhat of a misnomer, for they are never hollow, and in the same 

 way too the duct is solid when it is laid down, but a hollow appears 

 towards the front end during the second day. Its posterior end 

 grows back above the nephrotomes quite independently without 

 contributions from the posterior members of the series, until at 

 2 \ days it reaches and fuses with the cloaca. The lumen also spreads 

 slowly down it and reaches the end about the third day. This 

 Wolffian duct then is composed of two parts ; anteriorly it is a 

 fusion of portions of separate tubules, and posteriorly it is a con- 

 tinuous structure. The only indication of a cavity in the tubules 

 themselves is to be found at their lower end, where a few of them 

 contain short hollows continuous with the ccelom, and this hollow 

 is taken to represent the vestigial nephrotome. Furthermore, no 

 indications of a Malpighian body are to be found in any of them. 

 Just after the appearance of the tubule bud, the cells of the 



