(ii. XT] EFFECT OF INJURING A BLASTOMERE 113 



embryos that he obtained by Roux's methods, and contrasts 

 these with those described above. Some of the embryos showed 

 ;he condition of spina bifida, i.e. with both sides of the body 

 Leveloped and with a large yolk-exposure in the mid-dorsal 

 } Others of the embryos were only slightly injured by 

 "he operation and developed nearly normally. In these the en- 

 ire dorsal region was well developed and the blastopore closed 

 :o a small ring. Only on the ventral side was a small defect 

 found where the outer and middle germ-layers were absent. 

 in. these latter embryos, and in those showing spina bifida, 

 [ertwig believes the injured blastomere was not killed or 

 even sufficiently injured to prevent its partial development. 

 That this is the true explanation cannot be doubted ; for it is 

 not at all unusual to find after the operation that the injured 

 blastomere may separate off small portions of itself as cells that 

 develop along with the cells from the uninjured half. Here, it 

 seems to me, is the uncertain part of Hertwig's work. He has 

 not observed, as far as stated, the segmentation of each egg on. 

 which he has operated, and consequently his results are open to 

 the objection that in many cases, where he does not suspect it, 

 the injured cell has also continued to divide and to form a part 

 of the later embryo. 



In nearly all of the embryos described by Hertwig the 

 medullary folds are unequally developed. 2 Hertwig's attempts 

 to meet this fact do not seem to me altogether satisfactory. A 

 large number of the embryos have developed unsymmetrically. 

 The ventral and posterior yolk-mass lies higher up on one side 

 than on the other. In consequence of this, one side of the 

 medullary fold lies nearer to the injured yolk than does the 

 other, and as a result the two sides of the body are unevenly 

 developed. The asymmetrical position of the blastopore on 

 the living part is assumed to be the underlying cause of the 

 later asymmetrical position of the medullary folds; but for 

 the primary reason of the lack of symmetry of the blastopore 

 itself Hertwig gives really no explanation, and to state that it 



1 Among these embryos Hertwig describes one that seems to have been an 

 excellent example of Roux's " hemiembryo lateralis." 



2 There are a few exceptions. 



i 



