Hyatt.] 144 [March 5 



zoa, which has, we think, great significance. 1 They have a pro- 

 longed gestation which can be compared with the similar prolon- 

 gation of this period in the development of the ovum in the Met- 

 azoa. The reproductive bodies of all kinds, whether asexual or 

 sexual, are retained within the body for a comparatively pro- 

 longed period, during which they undergo division and attain 

 very nearly the adult structures and aspect, according to Biitschli, 

 and acquire cilia before they become free. They, in fact, attain 

 the age at which they compare quite closely with the locomotive 

 embryos of Porifera and many other types of the Invertebrata 

 before they become free. 



The whole process of segmentation takes place under condi- 

 tions which effectually protect the earlier stages just as it does in 

 the Porifera and in the higher Metazoa. The action of protection 

 as correlative w r ith concentration, and the consequent suppression 

 of the more indirect modes of development of the ancestral types? 

 and the subsequent influence of protection in helping heredity to 

 maintain uniformity in the embryo of the tyj)e, have been discussed 

 by the author in "Genesis of'Planorbis at Steinheim," and also in 

 "Fossil Cephalopods of Mus. Comp. ZooL" (Proc. Am. Ass. Adv. 

 Sci., vol. xxxii, p. 232. 



Balfour also subsequently advocated the same idea, but attribu- 

 ted concentration, or, as he called it, abbreviation, to the action of 

 protection, whereas we regard the two things as correlative, 

 since protection often does not exist when concentration takes 

 place. It needs only be said here that protection of the embryo 

 often appears to arise simultaneously with concentration, and to 

 occur through some change of habit or habitat. Such changes 

 are in our view reactions of the organism, due to its efforts to 

 meet the requirements of changed or new surrou 1 lings by modi- 

 fications of its acquired structures and organs partly according to 

 Dohrn's hypothesis, and partly according to Ryder's theory of 



1 We cannot see why their sexual reproductive bodies cannot be designated as ova 

 and spermatocysts. They are ova ai d spermatocysts in chaiacteristics and structure, 

 and must be considered as essentially the same. Volvox itself is, therefore, in reality 

 a true egg-bearing animal withcnly one layer to the body, and must, therefore, be con- 

 sidered a true Protozoan; but it differs only from an embryo sponge in respeot to the 

 number of layers in the body, and the ordinarily accepted difference between Metazoa 

 and Protozoa breaks down in the effort to find the differences between its sexual zoons 

 and those of the Porifera. 



