OF ARTS AND SCIENCES. 69 



takes place before the circumcrescent growth of the blastoderm is half 

 completed, must be accounted for on the same general grounds that 

 we should account for the formation of the embryo before the closure 

 of the blastopore. From this standpoint, the fact that the periblast 

 accompanies the blastoderm around the yolk becomes comprehensible 

 and reconcilable with our general interpretation. 



The investigations of Gotte make it sufBciently clear that a similar 

 ingrowth is characteristic of other meroblastic ova ; and, contrary to 

 the statement of Balfour (1. c, p. 69), we are confident that the de- 

 velopment of the amphibian and the elasmobranch ovum furnishes 

 nothing incompatible with this fact. The counter arguments drawn 

 from this source will be considered in the memoir that is to follow 

 this paper. 



The epidermal layer of the ectoderm takes no share whatever in 

 the involution. The entoderm bends directly into the deeper layer of 

 the ectoderm, as is shown in Fig. 6, and as has been stated by Hoff- 

 mann and Henneguy. But this point can only be determined by sec- 

 tions ; and this accounts for the error into which Kingsley and Conn 

 have fallen, in supposing that the epidermal layer alone is inflected 

 (1. c, p. 201). 



An optical section, coinciding with the future median plane of the 

 embryo, a few moments after the first indications of the ring appear, 

 shows that the involution is not equally strong at the two opposite 

 points of the ring. At the posterior margin, the in-rolling portion 

 presents a strongly voluted outline ; while at the anterior border it is 

 much more feebly expressed. As the ring widens ceutripetally, we 

 notice that the posterior thicker portion flattens and thius out as it 

 spreads inward to form the " embryonic shield." The inward growth 

 of the ring is completed in about an hour at all points, except at the 

 posterior border, where the " shield " still continues its centripetal 

 growth. Very soon after the ingrowth, which we may call the ento- 

 dermic ring, to distinguish it from the ectodermic portion of the em- 

 bryonic ring, has fairly begun, it appears everywhere to be only one 

 cell thick, except in the axial region of the shield, where we find it 

 from two to four cells deep. During the second hour of the ring, the 

 shield, which represents the anterior end of the embryo (the hind end 

 being represented prospectively by the remainder of the ring), becomes 

 considerably thinner and nearly doubles its axial length. At the end 

 of this time, it is only about three cells thick where it bends into the 

 ectoderm ; and from this point it becomes gradually thinner towards 

 its anterior free edge, where it is only one cell thick. 



