No. I.] THE EMBRYOLOGY OF CREPIDULA. 20I 



cocity produce more perfectly and more surely the adult organ- 

 ism. The percentage of abnormal forms among animals which 

 show no precocity is no greater than among those with pro- 

 nounced precocity. (See abnormalities of development among 

 gasteropods, p. 30.) 



(4) It seems probable that by a shortening of the process of 

 development there would be a distinct saving of energy; for if 

 we regard only the energy expended in nuclear and cell division, 

 it is possible to see that in an organ which reaches functional 

 activity after a dozen divisions less energy has been expended 

 than in one which reaches this stage only after one hundred 

 divisions. To this saving of energy precocious segregation tnay 

 in general be due. 



The " reflection " of similar larval or adult characters would 

 produce similar effects upon different eggs, and consequently 

 the similarity of the prelai'val stages of annelids and mollusks 

 may be held to be due to the similarity of their latvae; but there 

 is no reason for supposing that this parallel precocity has been 

 independently acquired by annelids and mollusks, since it may 

 well have been produced before the phylogenetic separation of 

 those groups. 



(b) Regional Homologies. 



It is certain that a considerable number of accurate cell 

 homologies are found among annelids, lamellibranchs, and gas- 

 teropods, but such homologies cannot at present be claimed for 

 all the cells of the cleaving eggs of these animals ; and between 

 these and other groups which manifest determinate cleavage, 

 e.g., Turbellaria and Ctenophora, it is probable that no such 

 accurate cell homologies exist. As has been argued elsewhere, 

 one ought not to expect more complete homologies among blas- 

 tomeres than among organs. In most cases, however, which 

 have been carefully investigated, homologous organs come from 

 the same regions of the cleaving &gg. This is a fact of the most 

 general application and of the greatest importance. Apical 

 sense organs, cerebral ganglia, and the ectoderm in general 

 come from the animal pole, the entoderm comes from the vege- 

 tal pole, while the mesoderm usually comes from the region be- 



