MATERNAL INHERITANCE 231 



In cases in which a foreign sperm may start develop- 

 ment but take no further part in it, the resulting' embryo 

 is like the maternal race. Here we are dealing not so much 

 with maternal inheritance, but rather with a special kind 

 of parthenogenesis. Such eggs, however, rarely go beyond 

 the cleavage stages. 



The rate of cleavage of an egg fertilized by foreign 

 sperm usually coincides with that of the species to which 

 the egg belongs. Since the cytoplasm of the egg has, prior 

 to fertilization, always been under the influence of its OAvn 

 nucleus, this relation is what might be expected. It is 

 necessary to study eggs from an i^j generation in such 

 cases in order to judge how far paternal chromosomes 

 may influence the cleavage. It is thinkable, for example, 

 that a spermatozoon might bring in a factor dominant for 

 rate of cleavage, but because this factor had not had time 

 to influence the cytoplasm its effect would not show in the 

 Pj cross. In the F^, on the other hand, the paternal char- 

 acter might prove dominant. Both Driesch and Boveri 

 have shown in the sea urchin that the rate of cleavage, 

 the pigmentation, and the kind of gastrulation are entirely 

 or largely determined by the egg — they differ in opinion 

 only as to how soon the influence of the sperm can be seen. 



At the time w^hen the larval skeleton is formed most 

 observers agree that the influence of the foreign sperm 

 makes itself felt. Most of the accounts of the skeleton of 

 hybrid sea urchins describe it as intermediate in struc- 

 ture, but one that varies widely under different external 

 conditions. Tennent has shown, in fact, that the character 

 of the hvbrid larval skeleton is so greatly influenced by 

 the alkalinity or acidity of the sea water that it can be 

 artificially thrown towards one or the other side — mater- 

 nal or paternal. Loeb, King and Moore have attempted to 

 determine whether the larval skeleton has dominant char- 

 acters in certain parts and recessive ones in other parts. 

 They crossed the sea urchins, StronrjylocentroUis Francis- 

 canus and S. purpuretis. Both the straight cross and its 



