Ch. XI] EFFECT OF INJURING A BLASTOMERE 119 



first cleavage. This depression or groove on the surface may 

 divide at eitlier end into. two distinct and independent grooves. 

 Cross-sections through such an egg show tliat the groove on 

 the surface is the result of an invagination to form an archen- 

 teron in each half. This means that each half-blastula has 

 begun to invaginate along the common line of contact of the 

 halves. Since the halves are in contact, the overgrowth of 

 each blastopore is impossible. The lips of the blastopore of 

 each half, therefore, have extended around the equator of the 

 egg as in the spina-bifida embryos. A medullary fold appears 

 later along each blastoporic rim, and then it becomes apparent 

 that two embryos are present, each a spina bifida, and united 

 by a common central yolk-mass (Fig. 30, C, D, E). The open 

 dorsal surfaces of these two embryos are turned toward each 

 other (Fig. 36, E). 



This seems to be the more common type of double monster 

 produced from these eggs. If, however, the blastoporic invagi- 

 nations begin at different regions of the two hemispheres, many 

 possible variations of the method described will be introduced ; 

 Schultze and Wetzel have in fact, as we have seen, described 

 several forms of these double monsters. (Fig. 36, B, F.) 



It seemed to me not improbable that Schultze's results 

 explain in part the difference in the results of the experiments 

 of Roux and of Hertwig. If, on the one hand, the uninjured 

 blastomere retain its normal position after the operation, 

 i.e.. with the black pole turned upward, then there should 

 develop a half-embryo, in Roux's sense. On the other hand, 

 if, after the operation, the position of the egg be reversed so 

 that the white pole of the uninjured blastomere is turned 

 upward, then a whole embryo of half-size might develop. In 

 Roux's experiment it is probable (although not explicitly 

 stated) that the black hemisphere always remained upward 

 after the operation. Hertwig does not say in what position 

 the eggs lay in his experiments. He only says that in the blas- 

 tula and gastrula stage the heavier injured yolk was down, and 

 the lighter uninjured blastomere was above. If, immediately 

 after the operation, the eggs lay with the injured blastomere 

 below, we should expect some change to take place in the 



