38 LIBBIE H. HYMAN. 



In the case of the cod, however, the state of affairs is the 

 reverse. The earliest stages examined were those consisting of 

 a small blastoderm composed of a considerable number of cells. 

 At this time the periphery is more susceptible than the central 

 portions. This is the case before the germ ring has become 

 visible. The peripheral portion of the blastoderm becomes 

 sharply separated from a central area and disintegrates long 

 before the latter. This condition is illustrated in Fig. 5, the 

 peripheral ring having disintegrated leaving a central disc still 

 intact. In very young blastoderms, the periphery is equally sus- 

 ceptible at all points, as in Fig. 5, but very soon a differential 

 susceptibility appears. One region of the periphery -is found in 

 the majority of cases to be more susceptible than any other part 

 of the blastoderm and from this point of high susceptibility the 

 disintegration proceeds in both directions along the periphery. 

 Two stages in the disintegration of such a blastoderm are repre- 

 sented in Figs. 6 and 7. There cannot be any reasonable doubt, 

 in view of the conditions at later stages, that this peripheral area 

 of high susceptibility is the place at which the embryonic shield 

 is to arise. It may therefore be stated that this region is physio- 

 logically different from the rest of the blastoderm long before its 

 morphological role becomes apparent. 



The metabolic conditions during the early blastoderm stages of 

 these three species of fish are therefore of two kinds. In the 

 case of Tautogolabnis and Fundulus, the central regions are 

 more susceptible while in the cod the peripheral region has the 

 highest susceptibility. There can be little doubt that these differ- 

 ences are the expressions of real differences in the physiology of 

 development of the two classes of eggs. This matter is discussed 

 further below. 



3. Disintegration of Later Stages of the Blastoderm. As the 

 blastoderm of Tautogolabnis expands over the yolk, the region of 

 high susceptibility comes to lie more posteriorly, that is, in that 

 portion in which the embryo is to appear. The posterior half of 

 the blastoderm is markedly more susceptible than the anterior 

 half, as shown in Fig. 8. This whole region appears as far as 

 could be determined to be equally susceptible throughout; from 



