180 



INTERNAL FACTORS 



IV. i 



A blastomere segments like a whole ovum, except that the 

 second division may be unequal, and gives rise to a normal 

 blastula, gastrula, and embryo of half the normal size; f blasto- 

 meres behave in the same way (Fig. 89). Incomplete separation of 

 the cells after the first or second division leads to double embryos 

 in the first case, double, triple, or quadruple embryos in the 

 second. The double embryos may make any angle with one 

 another and continue to live for some time (Fig. 90). 



A ^ blastomere segments like a whole egg (the second division 

 is, however, always unequal) and gives rise to a normal blastula 

 and gastrula, more rarely to an embryo of one-quarter size. 



FIG. 90. Double gastrulae of Amphioxiis, from incompletely separated 

 i blastomeres. w 1 , w 2 , separate blastopores ; u, common blastopore. 

 (After Wilson, from Korschelt and Heider.) 



A | blastomere will segment normally, or nearly so. A blas- 

 tula is rarely formed, usually an open curved plate of cells, 

 which are ciliated and swim about but do not gastrulate. 



Lastly ^ blastomeres divide but produce only an irregular 

 heap of cells. 



It is obvious that the potentiality of a blastomere to form 

 a whole embryo diminishes with its germinal value, the ratio it 

 bears to the whole ovum; but it is not so clear whether this 

 diminished capacity is due to the lack of some necessary, specific, 

 organ-forming substance or merely to the small size and lack of 



