chorion, which were previously filled with blood, then 

 become abated. The chorion becomes separated from the 

 umbilicus, and the chick breaks out of the egg. 



The general characteristics of the third period of 

 development of the chick embryo Baer gave in his concluding 

 section (§ 16) of the first part. His first concluding 

 point leads to the assumption that during the third period 

 of embryonic development the embryo gains predominance over 

 the remaining parts of the egg. If at the beginning the 

 embryo represented a part of the rudiment, then the latter 

 now becomes a part of the embryo. The parts of the egg from 

 which the embryo became separated in the second period now 

 gradually become embedded in it. Baer identified the same 

 three stages of separation: the separation of the embryo 

 from parts outside itself, their retraction inside, and, 

 finally, life outside the egg, in which the animal is no 

 longer part of the egg, but becomes dependent on the outside 

 world. 



Another important point of the concluding paragraph 

 established that development represents a transition from 

 the general to the special. While in the second period the 

 embryo acquires features which are general to all vertebrates, 

 in the third period it reveals the particularities of birds 

 ("the chicken becomes a bird") in the features of its res- 

 piratory system, extremities (wings) and covers (feather 

 rudiments) . "However, from the beginning it is a bird in 

 general, and not a bird from the family of chickens." 

 Gradually, the features of land birds appear (shortening of 

 the interdigital web or membrane); then, the features of 

 chickens can be distinguished (form of the head, the separa- 

 tion of the glandular stomach from the gizzard, blunt claws 

 and scales above the nostrils) . At the time of hatching, 

 the generic feature appears (the comb on the head) while 

 the individual particularities develop outside the egg as 

 the chick reaches its maturity. The general conclusions given 

 here by Baer are very briefly illustrated in the second part 

 of the work. 



337 



