10 S. F. CLARKE. 



traces of division into tliree portions; the divisions making their 

 appearance first upon the ventral side and running at right angles 

 to the long axis of the lobes. In this way the three pairs of gills 

 are first indicated, and the divisions between the lobes are the first 

 external indications of the branchial clefts. The small rounded 

 lobes anterior to the gills have already become elongated and some- 

 what resemble their perfect form. 



The integument over the pericardial region has become so 

 transparent that the heart can be seen by transmitted light to be 

 pulsating. U]) to this period the embryos, since the closing of the 

 medullary folds, have been of a uniform dark brown or brownish- 

 black. Now, a number of large stellate cells filled with black 

 pigment make their appearance along the region of the protoverte- 

 brie, from the branchial lobes nearly to the anus; others soon make 

 their appearance in the same region filled with a greenish-yellow 

 pigment and some of the external epithelial cells have the same 

 yellowish-green hue. These pigment cells are very early found 

 upon the brachial lobes and soon extend overall parts of the body. 

 The body of the embryo is now straight and five or six vertebrae 

 have been formed in its rapidly developing tail. Plate 2, Figure 15. 



A dorsal view at this stage, or a little earlier than this, before 

 the divisions appear in the branchial lobes, shows the body of the 

 embryo resting on the unabsorbed yolk, of which there is still con- 

 siderable left. It also shows very well the relative position of the 

 eyes, balancers, branchial and brachial lobes, and the division 

 between the neck and body. This latter differentiation is now 

 becoming more and more evident. A ventral view shows that a 

 deep constriction has taken place on the sides of the neck, thus 

 marking off that region from the rest of the body. In the ante- 

 rior end of the body region, where it has been made narrow by 

 the lateral constriction, is the pericardial region ; the integument 

 is here so thin that the chambers of the heart may readily be dis- 

 tinguislied and the pulsations counted. 



The divisions of the branchial-lobes, or the branchiae, as we 

 may now call them, for the blood is by this time circulating in 

 them, and the balancers all grow rapidly in length. The caudal 

 portion of the body also becomes longer, but otherwise there are 

 but few external changes posteriorly, for a day or two. Most of 

 the energy seems to be devoted to the growth of the branchiae and 

 the balancers. In examining a large number of specimens, it is 



