No. 2.] FORMATION OF THE FISH EMBRYO. 423 



since the area of the embryonic region has also increased, it 

 must have received cells from the extra-embryonic region. 



The quantitative relations of the embryo at different periods 

 of its formation may be next discussed. The series of surface 

 views drawn in Figs. 1-8 give the principal stages of Ctenola- 

 brus. The embryonic region is well defined in Fig. 2 (16 hours 

 old), and the head end has extended to the central point of the 

 earlier blastoderm. The head does not now, however, quite 

 reach the center of the present blastoderm. This is due, I 

 believe, to the quicker thinning out of the side of the blastoderm 

 opposite to that side where the embryo is forming, so that the 

 germ-ring is here pushed further away from the earlier center. 



In the next figure. Fig. 3, A, the broad embryonic region 

 is more sharply marked off from the germ-ring. A side view 

 of the embryo is shown in Fig. 3, B (18 hours old). 



At the next stage (Figs. 4, A, B) the embryo shows a marked 

 axial concentration. In the middle line and anteriorly the 

 embryo is thicker than elsewhere. A little more than half the 

 yolk has been covered by this time (20 hours old). 



A side view — optical section — of an embryo 2i>^ hours 

 old is shown in Fig. 5, A. The germ-ring of this embryo is 

 shown in Fig. 5, B, in surface view. The posterior end of the 

 embryo is shown in Fig. 5, C. These figures show that both 

 an elongation of the embryo and a further axial concentration 

 have taken place. The side view shows that the embryo has 

 also increased in depth (dorso-ventral). The relations that the 

 axial concentration, elongation, and increase in depth bear to 

 one another will be discussed later. 



Finally, the form of the embryo when the germ-ring has 

 almost closed is shown in side view in Fig. 6, A, in surface 

 view in the three pieces of Fig. 6, B, and the posterior-end and 

 germ-ring in Fig. 6, C (24 hours old). 



Comparing these latter figures with the preceding stage, we 

 see that a marked elongation has taken place. The surface 

 views show, however, that the embryo is nearly the same width 

 in both stages (Figs. 5 and 6), i.e., the axial concentration has 

 about reached its limit. The head-end of the older embryo is 

 even somewhat wider than that of the younger, owing to the 



