development with the development of birds. The ova of 

 reptiles are supported by a small amount of albumen and 

 deprived of chalazae. Their development elapses more 

 slowly than in birds, so that the heart, for example, 

 although generally similar to the heart of birds, is delayed 

 in early stages of development; the same holds for the 

 development of extremities. The ova are laid when develop- 

 ment is sufficiently advanced. The development of the ova 

 outside the maternal body corresponds to the third period 

 of avain development. The difference between the embryos 

 of reptiles and birds are these: the vascular arches, 

 coming out from the aorta, are longer in lizards and snakes 

 than in birds, and their aorta in the post-embryonic period 

 retains two roots, from which the right is much larger than 

 the left. The cardiac chamber remains without septum. Thus, 

 Baer concluded, "with respect to circulation, the reptiles 

 remain in an embryonic condition, so the circulation system 

 remains in an incomplete double form. In contrast, most 

 birds do not acquire external genital organs. In this regard, 

 the birds thus remain in a more embryonic condition in 

 comparison with reptiles" (II, 8b. 213 (141)). In the 

 development of viviparous snakes and lizards Baer did not 

 notice any essential peculiarity. 



Turning to the development of mammals (§ 9 (164) and 

 10 (233)), Baer first noticed that in different representa- 

 tives of his class, the moment of birth coincides not with one 

 or another stage of embryonic development, but in accordance 

 with the fact that there can be both early and late-born 

 mammals. Immediately after birth in the former, the young are 

 incapable of independent movement, while in the latter the 

 young are actively moving. "The early-born mammals," Baer 

 wrote, "are, thus, transitory forms, and the late-born mammals 

 constitute a proper branch of this class" (II, 9a, p. 218 

 (164)). The development of early-born mammals Baer did not 

 study himself. Rather he referred to the limited literature 

 of his time on monotremes and marsupials. 



Concerning the late-born mammals, Baer first noticed 

 great differences in the external form of the embryonic ova^ 



3. Embryonic vesicles, by recent terminology. 

 406 



