THE THIRD SOMITE. 75 



ccEloraic, but my account of the geuerative organs and their 

 ducts was correct. I expressly stated in that paper that I had 

 not succeeded in following the later changes in the somites. 

 My words were as follows : 



" So far the development of the somites is quite clear and easy to follow. 

 But the changes by which the dorsal divisions of the somites are converted 

 into their permanent form take place at a late period of develnpmeut^during 

 November — and are, in consequence of the thinness of the walls, extremely 

 difficult to follow. I have not succeeded entirely in following them. I will 

 content myself, therefore, with making the following statement, of the truth of 

 which I am by no means confident. The dorsal divisions unite with each other 

 transversely and longitudinally, and give rise to a continuous cavity — the 

 pericardial cavity. The portion of this cavity containing the generative cells 

 become separated from the rest as two tubes which form the generative glands 

 and part of their ducts, and come to lie ventral to the pericardium in the 

 central compartment of the body cavity. The external parts of the generative 

 ducts appear to be derived from the modified leg cavity of the anal papillae." 



With regard to previous observers of the adult anatomy, 

 Balfour — the discoverer of the nephridia — overlooked the small 

 vesicle in the leg cavity into which the funnel of the nephri- 

 dium opens. This oversight is not to be wondered at in the 

 case of the species (cape n sis) which he worked at, for the 

 vesicle is extremely difficult to see in the adults of Peripatus 

 capensis. Gaffron — the discoverer of the pericardial cavity 

 and cardiac ostia — similarly overlooked the vesicle, though 

 it is quite distinct and easy to see in the species — Peripatus 

 Edwardsii (?)— which he examined. 



Description op the Development of the Mesoderm with 

 ITS Cavities from Stages d — f. 



It will facilitate matters if I begin by describing the deve- 

 lopment of a particular pair of somites and of the mesodermal 

 structures around them, and I will take for this purpose the 

 third somite of the body (somite of the oral papillae) as one 

 which is typical of the rest and in which the changes are most 

 easily followed. 



