ORGANS IN THE EMBRYO OF THE FOWL. 93 



embryonal differentiation, for any process of self-differentiation 

 of a structure might be analyzable into correlative differentiation 

 of its parts. 



For the development of the higher animals at least the extra- 

 organic environment is non-determinative. The development of 

 the ovum as a whole is therefore a process of self-differentiation. 

 But it is usually assumed that it is otherwise with the differen- 

 tiation of its constituent parts ; the extreme view being that each 

 influences the mode of differentiation of all the remainder. From 

 this standpoint the complexity of the correlative processes of 

 differentiation must increase in proportion to the increase in com- 

 plexity of structure. 



Theoretically, at least, the determinative value of correlative 

 differentiation in any case may be (i) absolute, /. c., the mode of 

 development of a part being determined entirely from without 

 itself; (2) partial ; (3) wanting, /. c., absolute self-differentiation. 



Our present knowledge is enough to exclude the first theoreti- 

 cal possibility. No principle in embryology is better established 

 than that sooner or later the embryo is a mosaic of embryonic 

 rudiments, each of which is to a certain extent self-determining. 

 This mosaic of rudiments may become visible very early, as in 

 those ova exhibiting a definite cell-lineage of organs, or it may 

 appear later. In some cases, at least, the unsegmented ovum 

 itself is a simple mosaic (ovum of ctenophores according to 

 Fischel ; ovum of Unio, Lillie ; ovum of sea-urchins, Boveri ; 

 ovum of frog, Roux, Schulze and others). Indeed it is quite 

 probable that all ova are more or less simple mosaics of embry- 

 onic rudiments. 



Unless, therefore, we wish to beg the entire question we must 

 proceed on the second hypothesis. This is the writer's stand- 

 point, and the problem is to determine as many definite correla- 

 tions as possible and to investigate their nature. 



There is probably no conception in embryology so vague as that 

 of correlative differentiation, as the following citations may serve 

 to show : 



Hertwig : " Zelle und Gewebe," II. : 



" Die Wechselwirkungen (Correlationen) zwischen den Zellen 

 eines Organismus und ihren Derivaten bilden sich mit dem 



