No. I.] THE EMBRYOLOGY OF CREPIDULA. 199 



Wilson ('92) to precocious segregation. Lillie ('95) maintains 

 that it is parallel precocioiis segregation that conditions cell 

 homologies. What the cause of this parallel precocious segre- 

 gation, or of precocity in general, may be is a matter of much 

 doubt. 



The term precocious segregation was first introduced by 

 Lankester ('77), to indicate the fact that the segregation of 

 parts or layers might be "pushed back into the egg." From 

 the expressions which are frequently used in this connection, 

 such as the "pushing of characters back into the Q-gg^ "the 

 reflection of adult characters back upon the egg," etc., it seems 

 that the process is commonly considered a direct rather than 

 an indirect one; or, in other words, that adult characters appear 

 earlier in successive generations, owing to the influence of the 

 body plasm upon the germ plasm. This distinct form of 

 Lamarckism is apparently held by embryologists who repudiate 

 that doctrine in any other form; it is, however, as can be seen 

 by a moment's thought, the very centre and stronghold of the 

 Lamarckian doctrine. On the other hand, it is possible to 

 explain precocity in development by assuming that eggs show 

 multifarious variations, and that natural selection has picked 

 out such as are most beneficial to the species. In fact, there 

 is no doubt that eggs show repetition and variation phenomena 

 as truly as do adult organisms, and they would therefore afford 

 a field for the action of natural selection. 



No satisfactory or conclusive evidence as to the cause of 

 precocity can at present be furnished, but the following obser- 

 vations may help to an ultimate solution of this problem : 



(i) Adult characters have influenced embryonic characters, 

 and especially cleavage stages, more than the latter have influ- 

 enced the structure of the adult. This principle finds very 

 many illustrations, among which may be mentioned the follow- 

 ing: great individual variations of cleavage produce slight, if 

 any, variations in the adult, as is shown normally in the case of 

 Renilla and Amphioxus (Wilson) and experimentally in the case 

 of many different animals (Driesch, Hertwig, Wilson, Morgan, 

 etc.). Many determinate characters of cleavage, which can 

 have little or no significance for the o^gg itself, are yet of 



