40 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Stage II. 



Embryos of Stage II have increased considerably in length over those 

 of Stage I. The head and tail are clearly marked off from the body. 

 The optic vesicles still communicate widely with the forebrain, and there 

 is no sign of a lens-thickening. 



The chief difference in the structure of the somite between this and 

 the previous stage, as may be seen from Figure 3, is a great increase in 

 its dorso-ventral diameter. Its lumen in that diameter has increased even 

 more than has the somite, but it has decreased in its antero-posterior 

 diameter, so that the anterior and posterior walls of the somite now 

 almost touch each other. Figure 3 does not fully show this increase in 

 ^he vertical diameter of the luiuen, because the somites are oblique both 

 to the frontal and slightly to the sagittal plane of the body, their ante- 

 rior and posterior walls sloping backward both dorsally and laterally. 

 Sections in the three planes, transverse, sagittal and frontal, show the 

 anterior, posterior, and median walls of the somites to be thick, the ven- 

 tral and dorso-lateral walls thin. The somites are more closely applied 

 to each other than in the preceding stage, and fit together like opistho- 

 coelous vertebrae. The lateral mesoderm is more distinctly differen- 

 tiated from the somite than in Stage I. 



The duct has reached somite 14. Germ cells are present and rather 

 conspicuous opposite somite 11. They are differentiated out of ihemost 

 doi'so- median portion of the lateral mesoderm ^ by the increase in size of 

 cells of both splanchnoderm and somatoderm (cl. g., Fig. 5, Plate l). 

 These cells form a rod parallel with the lateral edge of the somite and 

 enclose a small lumen which is a portion of the coelom. They are still 

 connected with the rest of the lateral mesoderm and with the mesomer. 

 At va. S7ig. (Figure 3) are seen cells which apparently give rise later to 

 blood-vessels. Their origin is unknown. 



Sta/je III. 



The embryos of Stage III possess about twenty somites and measure 

 from 5 to 5.5 mm. in length. The fundaments of the gills are begin- 

 ning to show as external protuberances. The cavity of the optic vesicle 

 is nearly obliterated, and the lens-thickening is beginning to appear. 



A comparison of Figure 13 (Plate 2), with Figure 3 shows the main 

 points of advance in the development of the somite between this stage 



1 This tissue represents, of course, the hypomer, but segmentation is rather 

 indistinct and very transitory. 



