MUSEUM OF COMPARATIVE ZOOLOGY. 213 



anterior somite is still present, and a few more posterior protovertebra; 

 are in process of formation. .Frontal sections just ventral to the chorda 

 are very instructive. By following through a series of these, an idea 

 can be had of the successive changes which take place in passing from 

 the protovertebrae to the lateral plates, — a region of prime importance 

 for problems respecting the development of the urogenital organs. In 

 sections approximately tangent to the chorda at its ventral border (com- 

 pare Fig. 5), the plane of the section passes through the ventral floor of 

 the protovertebra, and cuts the somatic mesoderm near the place where 

 the protovertebra passes into the lateral plate. The body cavity is ex- 

 panded in the anterior part of the trunk. The mass of tissue on the 

 median side of the body cavity appears very broad, owing to the circum- 

 stance that the plane of the section, as before noted, lies in the floor of the 

 proto vertebra. The somatic layer is several cells thick, and very com- 

 pact in structure, owing to the fact that the section passes through the 

 dorsal margin of the pronephric thickening. In following the series of 

 sections farther ventrally, the boundaries between the segmental con- 

 stituents of the pronephric thickening become indistinct ; and in a sec- 

 tion 90 fi farther ventral they have wholly vanished. This section, 

 however, still shows traces of segmentation in the splanchnic layer, 

 which is here reduced in thickness, the plane of this section having 

 passed ventral to the floor of the protovertebra. Still farther ventrally 

 the segmentation of the splanchnopleure likewise vanishes, and finally 

 the pronephric thickening gives place to undifferentiated somatopleure. 

 I have looked in vain for prolongations of the body cavity into the prone- 

 phric mass at this stage. I believe that the pronephric thickening is to 

 be regarded as a solid proliferation of the somatopleure, in which, how- 

 ever, the somatic layer of the protovertebra? takes some part. 



Stage III. 



Plate I. Fig. 6. Plate II. Figs. 11, 12, 15-17. 



In embryos of this stage the medullary canal is wholly closed, the fun- 

 daments of two pairs of gills are present, and the auditory vesicle consists 

 of a shallow depression of the deep ectoderm. 



The pronephric thickening has now begun to assume a more definite 

 form, and during this stage becomes converted into a tubular organ. 

 I shall first consider the structure as seen in a series of cross sections 

 from an embryo measuring about 2.7 mm. in length. Figures 15 to 17 

 are from this series. The anterior end of the pronephric thickening is 



