244 BULLETIN OF THE 



involves the ventral portion of the lateral wall of the protovertebra itself, 

 although the greater part of the thickening is in the region of the lateral 

 plate. I have not been able to find any sharp plane of division marking 

 the lower limit of the thickening. The latter extends posteriorly through 

 a distance of three or four somites, but it is difficult to make out its rela- 

 tions to the protovertebne, in consequence of the small amount of differ- 

 entiation which these exhibit at this stage. It seems to me, however, 

 that the thickening reaches backward into a region posterior to that 

 in which the prouephric tubules later develop, and therefore represents 

 already the first fundament of both the pronephros and the anterior end 

 of the segmental duct. 



Frontal sections show the same relations between the pronephrio 

 thickening and the protovertebra) that I have described for Rana, but 

 in Bufo the coelom is entirely obliterated by the growth of the proue- 

 phric thickening, and consequently the prouephric chamber described in 

 a corresponding stage of Rana does not exist in Bufo. This circumstance 

 renders the determination of the precise boundaries between the two lay- 

 ers of mesoderm somewhat more difficult in the Toad than in the Frog, 

 but still there is usually an unmistakable line of division between soma- 

 topleurc and splanchnopleure even in the former. The pronephric thick- 

 ening at this stage is from two to three cells thick, and is a solid 

 mass. 



Stage III. 



In embryos of this stage, the fundament of a single pair of gill-folds 

 is present; the fundament of the auditory vesicle consists of a simple 

 thickening, which is just beginning to separate from the superficial ecto- 

 derm ; and five or six protovertebne have made their appearance. The 

 embryos measure from 2.25 to 2.50 mm. in length. 



The pronephric thickening becomes sharply marked off in this stage 

 from the undifferentiated mesoderm lying ventral to it, and the canali- 

 zation of the structure is accomplished by the arrangement of the cells 

 ai'ound a lumen. Segmentally, the pronephric thickening has in general 

 the form of a close fold of somatopleure, whereas intersegmental^ it ap- 

 pears as a flattened tube. The points of continuity with the coelom are 

 situated each directly beneath the middle of a protovertebra, and the 

 somites in which they appear are II., III., and IV. 



The duct arises as a backward continuation of the pronephric thicken- 

 ing, and contrasts very sharply in histological characters with the ecto- 

 derm, in consequence of the pigmentation and paucity of yolk spherules 

 in the latter. 



