THE EARLY DEVELOPMENT OF THE CHICK 231 



an important role in the modification of embryonic form, and 

 an essential role in relating the embryo to the walls of the mater- 

 nal cavity in which it develops, a relation that is singly the most 

 important distinction of Mammalian development. 



As an introduction to the subject we may outline, in a few 

 words, the more striking points in the life of the young chick. 

 Fertilization is internal, following a process of copulation, and 

 after laying, the eggs are brooded by the mother during their 

 three weeks incubation. This ensures, besides protection, the 

 temperature necessary for development. Normally this is 

 about 38 C.; development ceases entirely at temperatures 

 above 41, and becomes very slow when it falls to 25 or even 

 to 28. Newly laid eggs may remain alive, however, without 

 undergoing any advance, for a considerable period at much 

 lower temperatures than these; development then proceeds when 

 the temperature is raised. 



The processes of maturation, fertilization, cleavage, and 

 blastula formation are completed before the ovum leaves the 

 body of the parent, while the egg is passing down the oviduct. 

 Thus the hen's egg, as laid, may be roughly compared to the 

 seed of a plant, in which a simple embryo is already formed and 

 surrounded with nutritive material for its later development. 



The chief steps in the formation of the definitive embryo, 

 occur during the first day of incubation, and during the second 

 day a complicated series of folds appear, which largely effect a 

 separation of the embryo from the yolk-mass, with which it then 

 remains in connection by a narrow stalk. At the same time a 

 very extensive circulatory system develops, putting the embryo 

 into relation with its outlying food supply (Fig. 118). Develop- 

 ment now becomes very rapid since, through the morphological 

 separation of embryo and yolk, the usual retardative effect of 

 the latter is obviated, while an efficient physiological connec- 

 tion is established through the precocious appearance of the 

 circulation. 



During the second and third days of incubation, appear the 

 embryonic membranes and appendages, which provide for 

 respiration, extend the nutritive surfaces, and afford the spaces 



