90 The American Naturalist. [February, 



their ontogeny as larvae. It would, therefore, be superfluous 

 to linger for the defense of a view which is already accepted 

 by all biologists. 2. The embryonic development depends on 

 the presence of yolk. Now we have learned that the yolk has 

 developed very gradually and in all the lower animals appears 

 only in small quantities. It was not until the increase of 

 yolk material had become enormous, as, for example, in the 

 meroblastic vertebrates, that we find the development com- 

 pletely embryonic in type. With the increase of the yolk 

 comes the gradual transition from larval to embryonic devel- 

 opment. Since the embryo is dependent on the volk, and 

 since the yolk exists only in the higher forms insufficient 

 quantity, it follows that fully typical embryos can occur ex- 

 clusively in the higher (later developed) animal types. 



The fact that larvae represent the primitive forms of de- 

 velopment, obliges us to conclude that the correctnesss of 

 Weismann's theory of the continuity of germ plasm can be tested 

 better in larvse than in embryos, since in embryos the rela- 

 tions have undergone profound modifications by secondary 

 changes, which in this connection might easily deceive us. 



I do not venture to assert that I know what the present 

 form of Weismann's continuity theory may be ; I hold, how- 

 ever, the exact form of this much discussed theory to be non- 

 essential, because, according to my conviction, the theory can 

 in no form be brought into agreement with our present knowl- 

 edge. Nussbaum founded the theory, and opened the way 

 along which we certainly hope to make great advance. Let 

 me acknowledge the great value and the strictly scientific 

 character of Nussbaum's work ; doing this not merely because 

 I esteem it, but also because the unjust attempt has been made 

 to diminish his claim. Nussbaum 9 thought that the germ 

 cells are direct decendents of the fertilized ovum, keeping the 

 germinating power, while the rest of the cells developed from 

 the egg are transformed into the tissue of the body. He 

 brought forward several facts which could be interpreted in 

 favor of his theory. By this theory the whole problem of her- 



'■' M. Nussbaum. Zur I.>itl.>r<<n/.i<-nai ? ,1,-s ( i.-dilechts im Tirreieh \rch f 

 Mikrosk. Anatomie, XVIII, 1-121, (1880). 



