4O LIBBIE H. HYMAN. 



makes its appearance as a slight bulge on the germ ring as shown 

 in Fig. 12. At this time the region of the shield is the most 

 susceptible part of the embryo and from this region disintegration 

 spreads in both directions along the germ ring, as in Figs. 13 and 

 14. As the embryonic shield grows forward its anterior margin 

 is most susceptible and from this area disintegration extends 

 posteriorly in the shield as shown in Figs. 15 and 16. 



4. Disintegration of the Early Embryonic Axis. After the 

 germ ring has advanced half way or more over the yolk, the 

 embryo appears in the center of the embryonic shield. Two faint 

 lines outline its axis. In Tautogolabrus and Fnndulus the em- 

 bryonic shield is very faintly defined but in the cod is sharply 

 marked out on the blastoderm. In the two first-named species, 

 observations on the disintegration of the early embryonic axis 

 were much obscured by the occurrence of the shrinkage already 

 described. In but one or two cases in each species was the dis- 

 integration followed with certainty. In Tautogolabrus at the 

 time when the embryonic axis first becomes visible, the course of 

 disintegration is the following; the embryonic axis fades from 

 view and the region in which it lies undergoes disintegration. 

 Fig. 17 represents the normal blastoderm at this time together 

 with the yolk ; the embryonic side of the blatoderm is on the under 

 side of the yolk. Fig. 18 shows the same blastoderm much 

 shrunken, the yolk being omitted ; the area where the embryonic 

 axis was is seen to be in process of disintegration. As far as 

 could be determined the whole embryonic region is equally sus- 

 ceptible. In Funduhis an early stage of the embryo was observed 

 in but one case. In this individual two regions of high suscepti- 

 bility were observed, one at the posterior end, the other at the 

 anterior end of the embryonic axis. The first region was the 

 more susceptible of the two ; from these two points, disintegration 

 proceeded towards the middle of the axis. Three stages in the 

 disintegration of this individual are illustrated in Fig. 19. 



Early stages of the cod embryo were observed with ease. Dis- 

 integration begins at the anterior end of the embryonic shield and 

 proceeds posteriorly, more rapidly at the margins, as shown in 

 Figs. 20 and 21. In a slightly later stage, the susceptibility 



