MUSEUM OF COMPARATIVE ZOOLOGY. 24 



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older specimens, to be the first indication of the pronephric thickening. 

 A similar condition of the somatopleure is presented by embryos of Bufo 

 about 2 mm. long, in which the medullary folds are widely open. 



The general relations of the germinal layers at this stage are almost 

 identical with those in Rana, and the same histological criteria for distin- 

 guishing them can be employed. The ectoderm is very sharply marked 

 off from the mesoderm. The former is deeply pigmented, while the ad- 

 jacent mesoderm is almost destitute of pigment. The yolk spherules 

 of the ectoderm measure on the average abo,ut 2 /x ; those of the meso- 

 derm, about 4 p. 



In embryos in which the medullary tube is still widely open, the 

 somatopleure and splanchnopleure are separated from each other by a 

 distinct space, the coelom, which can be traced with perfect distinct- 

 ness into the protovertebral plate, where it becomes slightly expanded. 

 In the anterior half of the embryo, both the somatic and the splanch- 

 nic layers are only one cell in thickness. Posteriorly, and in the middle 

 trunk region, however, certain loose cells bordering on the coelom become 

 associated with the somatic layer ; but this layer is never, except at the 

 extreme hinder end, more than two cells in thickness. 



Stage II. 



Embryos in which the medullary tube is just closed exhibit a con- 

 dition of the mesoderm slightly different from that of Stage I. In the 

 posterior portion of the embryo, the mesoderm is quite thick in the re- 

 gion of the protovertebral plate, and becomes gradually thinner as it 

 approaches the ventral portion of the body. 



Anteriorly, the protovertebral plate shows traces of the differentiation 

 of four or five pi'otovertebrse. Of these, the most anterior lies in the same 

 transverse plane as the ganglion nodosum, and, following the method of 

 designation which was employed in the case of Rana, would properly 

 represent somite I. This protovertebra, as in Rana, shows signs of 

 transformation into mesenchyme, and is considerably compressed in the 

 region of the ganglion. 



The thickening has the general form which I have described for the 

 corresponding stage of Rana, and its anterior margin is situated under 

 somite II. 



In the region of its greatest thickness, which is somewhat lateral to the 

 boundary between the protovertebra and the lateral plate, it is two or 

 three cells deep. It thins out slowly on the ventral side, much more 

 rapidly on the side of the protovertebra, or dorsally. The thickening 



