230 OUTLINES OF CHORDATE DEVELOPMENT 



form; and from its study came many of the long dominant 

 conceptions of the process, as well as the morphology, of 

 development. 



To-day, when the development of the chick is better known, 

 as a whole, than that of perhaps any other vertebrate, it remains 

 an extremely important subject, not so much because of its 

 historical importance, nor because of its very great practical 

 convenience on account of its abundance, ease of manipulation, 

 and freedom from seasonal limitations in its use, as for other 

 more significant reasons. 



The development of the chick is fairly typical of the develop- 

 ment of the members of the largest and most important Chor- 

 date division, the Sauropsida. And besides, it suggests inter- 

 pretations of many of the very special features of mammalian 

 development. The egg of the fowl represents the climax of 

 the process of yolk accumulation, which begins in Amphioxus 

 and steadily increases through the Ganoids, Amphibia, and 

 Elasmobranchs. Consequently we find here, in pronounced 

 form, many interesting phases of the influence of deutoplasm 

 upon development. Indeed so great is the accumulation of 

 yolk here, that it remains no longer contained within the limits 

 of the embryo proper, and instead of exercising a retardative 

 influence upon the rate of development, it becomes 'so related 

 to the embryo that development is greatly hastened. In this 

 form the presence of the great mass of yolk results chiefly in 

 an extensive modification of the external form of the embryo, 

 particularly during its early phases. The embryo develops 

 for some time as a flat disc, upon the surface of the yolk mass, 

 so that it gives a sort of map-like spherical projection of a 

 Chordate embryo. And the morphological separation of em- 

 bryo and yolk, freeing the former from the influence of the inert 

 deutoplasm, enables the young chick to proceed to a remarkably 

 advanced stage of development during the three weeks of its 

 brief embryonic life. 



Moreover, the chick embryo possesses, in comparatively 

 simple form, certain embryonic membranes and appendages, 

 which, in the Mammal, are highly specialized and come to play 



