THE REDUCED SIZE OF THE YOLK SAC. 6 1 



nuclei, and as this nucleated area (in diameter) is measurably greater than that 

 of the yolk mass which the blastoderm comes to inclose, it follows that nuclei are 

 present in some of the outlying yolk masses. Of this, however, we must none the less 

 admit that no direct proof is at hand, since no sections of these outlying yolk masses 

 were made. In this connection I observe that if the embryo-bearing yolk mass 

 be examined even under a low power (plate viu, fig. 480) one obtains a fairly con- 

 vincing picture of its holoblastic character. 



(4) The foregoing evidence is none the less_strong if, conversely, we consider 

 that on no other morphological ground, save that of cleavage, using the word in a 

 broad sense, can this progressive and normal fragmentation be explained. 



Accepting, then, the premise that these divergent paths in the development 

 of the egg of Chimera took their origin in a holoblastic egg, the present con- 

 ditions may well have been developed on somewhat the following lines : In 

 the primitive Chimseroid the egg resembled that of Cestraciont; it was probably, 

 however, not as large as that of the recent Heterodontus, but its cleavage fissures 

 were deeper and more numerous. The embryo at that stage had the usual 

 external gills of the selachian. The next stage would be attained when the 

 gill filaments, passing beyond the stage of the well-known trophonemata, came to 

 appropriate the white of the egg which was contained in the deep cleavage fissures, 

 a process which in time caused or accompanied () the deepening of the fissures, 

 and in further time (fr) a rupture at the bottom of the fissures. Through such a 

 process yolk material came to escape and mingle with the albuminous contents of 

 the deep fissures. Such a process, we may now assume, was naturally followed by 

 adaptative changes in the trophonemata, which in the end accelerated the growth 

 and differentiation of the embryo. In short, at this evolutional stage the embryo 

 was receiving through a (morphologically) indirect channel an amount of nutri- 

 ment which rivaled that derived from the vitelline circulation. The result was 

 what one would have anticipated, i. e., the down growth of the vascular blastoderm 

 was retarded, while the fissuring of the yolk-mass became deepened and the 

 trophonemata further modified. The line of evolution thus carried on in the egg 

 will be seen to involve the fate of the yolk sac, viz. , in determining how great an 

 amount of the yolk could be diverted from it. In the present species (C. colliei) 

 about nine-tenths of the egg has been diverted, while in the Callorhynchids, where 

 the yolk sac is known to be larger, possibly not more than half. 



In the foregoing process it is suggested that the first steps in the disinte- 

 gration of the yolk mass were found in cleavage phenomena. It should, however, 

 be admitted that the cleavage may not have been equivalent to that of the usual 

 holoblastic type. The nuclei which spread peripherad may have been sperm-nuclei ; 

 and in this event the peripheral furrows are special phenomena, unconnected, 

 possibly, in phylogeny with the cleavage lines in the holoblastic egg. Certainly 

 in favor of such an interpretation is the fate of the disintegrating yolk masses, 

 since such a fate is paralleled somewhat by the sperm-nuclei in the shark egg. It 



