192 Biology in America 



of the egg, which cannot be mechanically rearranged by 

 centrifuging or otherwise. 



The foregoing experiments seem to show conclusively that 

 the developing animal is in some sense at least preformed 

 in the egg. No less conclusive however is the evidence of 

 a directly opposite character. In Amphioxus and many 

 Hydromedusae isolated cleavage cells give rise to complete 

 though dwarf larvae, while on the contrary it has been pos- 

 sible in some cases (Ascaris, Sphgerechinus) to produce nor- 

 mal, though giant larvae, by the fusion of two eggs or embryos. 

 Many intermediate forms exist between those eggs in which 

 one of the cleavage cells produces a partial, and those in which 

 it forms an entire larva. In some cases, as for instance in 

 certain echinoderms, an isolated cleavage cell may undergo 

 at first a partial development, but later a process of regula- 

 tion may ensue, resulting in the formation of a complete 

 larva. Different results may be obtained in the same animal, 

 depending on the method of experimentation. Thus if one 

 of the first two cleavage cells of a frog's egg be destroyed 

 with a hot needle and the egg left in its normal position a 

 half embryo results, but if the position be inverted a whole 

 embryo develops in the majority of cases. 



In this maze of conflicting evidence a final word can scarcely 

 be spoken. Undoubtedly different eggs differ in the extent 

 of their organization. If a part of the egg of the nemertine 

 Cerebratulus be removed prior to fertilization, no disturbance 

 of development ensues. If the two cells of the first cleavage 

 are separated, they undergo for a time a partial cleavage, but 

 very soon the normal development is resumed. But if one 

 of the four cells, resulting from the second cleavage, be iso- 

 lated, partial development proceeds for a longer time than 

 in the preceding case, the normal process not being resumed 

 until much later. We find here a possible explanation of the 

 divergent behavior of different eggs. In some the embryo 

 may be preformed in the egg, in others only in later stages 

 of cleavage. 



The phenomena of regeneration speak strongly for the 

 uniformity of both egg and adult. If the parts of the or- 

 ganism are predetermined in the former, then when one of 

 these parts is lost its replacement should be impossible; but 

 if the egg be isotropic (one part the same as another), and if 

 this uniformity persist in the adult, then a lost part should 

 be replaceable. 



The ability of regeneration in many animals has long been 

 known, being mentioned by Aristotle and Pliny. In the mid- 

 dle of the eighteenth century, the famous work of Trembley on 

 Hydra attracted widespread attention and several workers 

 entered this field. 



