the rounded form into a bean-shaped form. Histological 

 differentiation occurs there also, but not in the form of 

 vesicles as in the ovaries but in the form of small canals, 

 whose terminal parts go through the external layer of the 

 primary kidneys and reach the excretory duct. 



The essay on bird development Baer terminated with a 

 short summary. There he gave the differences between the 

 two aspects of the individual development, the development 

 for oneself and development for the species. The first 

 begins early but quickly progresses, while the second 

 begins only near the end of development of the individual, 

 and then is renewed annually. At the time of embryonic 

 development in the egg, vital functions differentiate by 

 the contents of the egg taking their origin from the 

 maternal body. But after release from the egg membranes, 

 the organism enters into an interaction with the external 

 world. In development of the methods of nourishment, it is 

 possible to distinguish three periods: the use of the yolk, 

 then the fetal fluid and finally nourishment from the 

 external world. The four periods of respiratory and 

 circulatory development Baer designated by the terms: 

 blood formation, simple circle, incomplete double and 

 complete double circle of blood-circulation. These periods 

 of development follow each other, and in each period one 

 can observe "preparation for what is produced in the 

 subsequent periods" 9 (II, III, 7u, 153). 



9. To Baer belongs, in this way, the first attempt to 

 divide chick development into periods depending upon 

 the different interrelations with the conditions of 

 the surrounding medium, in particular depending on 

 the character and sources of nutrition. 



404 



