210 BULLETIN OF THE 



reached no conclusions respecting the number and position of the head 

 somites, and since great diversity of opinion exists in the accounts to 

 be found in the literature, I shall make no attempt to number the 

 protovertebra? with which I shall have to do in any other way than 

 by beginning with the most anterior that is readily distinguishable. 

 Disregarding, then, the one which is wholly broken up into mesenchy- 

 matic tisue, somite I. lies in the same transverse plane as the fun- 

 dament 1 of the ganglion nodosum, and extends backward to the hinder 

 end of that structure. This protovertebra also shows signs of extensive 

 conversion into mesenchyme, although part of it at a later stage undergoes 

 muscular differentiation. Somite II. is the first of the series of well 

 developed trunk protovertebrse. In the specimen under consideration 

 somites I. to VI. are already differentiated. 



As I have stated, the somatopleure in the middle of the trunk consists 

 of a layer one cell deep, to which a few loose cells lying between it and 

 the splanchnopleure may possibly also be assigned. In the region of 

 somite IV. the somatopleure becomes thickened. The thickening is 

 greatest at the level of the lower margins of the protovertebne (com- 

 pare Plate II. Figs. 15, 16), and tapers both dorsally and ventrally. 

 It is to be remarked in this connection that the protovertebra? are not 

 yet fully separated from the lateral plates ; but that in cross sections 

 through the middle of a somite, — i. e. midway between the anterior and 

 posterior faces of a protovertebra, — the coelom can be traced to the 

 dorsal margin of the protovertebra, and furthermore that the somato- 

 pleure and splanchnopleure are seen to be continuous with the somatic 

 and splanchnic layers of the protovertebne. The somatopleural prolif- 

 eration extends forward as far as the anterior face of somite II. The 

 cells in the thickening have a columnar shape, and are at least two deep. 

 In some sections I have observed, in addition, a third row of thin 

 cells next the body cavity. Near the ventral limit of the thickening 

 a nearly horizontal line of division in the substance of the thickening 

 can be observed. When seen in cross section, this line is slightly con- 

 cave above. It is here that ruptures produced by artificial causes are 

 likely to occur, and the line thus indicated marks, I believe, the lower 

 limit of the pronephros. The somatopleural thickening is the funda- 

 ment of the pronephros, and I shall call it in the following pages the 



1 In the following pages I shall use the word fundament as an equivalent of the 

 German expression Anlage, the term fundamentum having been adopted as the 

 basis for the lettering of the figures of such structures in the " Contributions " 

 from this Laboratory. 



