ASYMMETRICAL DUPLICITY IN THE CHICK. 101 



There are two features of the blastoderm which seem to 

 require further brief eomment : — 



a. The significance of the primitive streak-like character of 



Rudiment ft. 



b. The relatively great distance separating the two embryonal 



rudiments. , 



rt. Experimental work on the localization of the prospective 

 embryo upon the unincubated blastoderm «,nd the relations 

 of the primitive streak to the future embryo have shown 

 that the primitive streak furnishes material for the develop- 

 ment of those regions of the latter lying posteriorly to the 

 heart, while, on the contrary, " the material of the primi- 

 tive streak does not enter into the foruiation of the brain " 

 (Peebles, 12), 



But obviously it does not follow that the converse must 

 hold, i. e., that material from which the brain normally 

 arises, may not under certain circumstances have the power 

 of giving luse to a primitive streak-like structure or tissue. 

 Judging from the analogy of lower forms^ one might 

 expect that it would have this power, though it is important 

 to recollect that, as Child points out, the limitation of the 

 capacity of reconstitution, i, e., of the potentiality of the germ, 

 is more or lefis progressive from lower to higher forms. 



In any case, owing to the conditions under whicli the 

 development of the chick takes place, its experimental 

 demonstration would be a matter of extreme difficulty. 

 Nevertheless, the primitive streak-like nature of Rudiment ft 

 appears without doubt indicative of its physiological isola- 

 tion, ^. g., that it is developing, or '^endeavouring" to develop, 

 if one may be permitted this teleological but expressive 

 phrase, as a whole. 



For, to pursue the analogy with lower forms, gastrulation 

 and primitive-streak formation resemble each other in this, 

 that they are each, the one in the holoblastic, the other in 

 the meroblastie egg^ the earliest fundamental morphogenetic 

 process by which the long axis of the body is determined. 



Hence the earliest obvious act, by which a physiologically 

 isolated pai-t of a holoblastically developing embryo is seen 

 to be developing as a whole, is the more or less successful 

 attempt at gastrulation. 



In the same way the formation of a primitive streak-like 

 structure — however imperfect — by a group of blastomeres 

 upon a blastoderm must surely be regarded as indicative of 

 their physiological isolation, that is, of an attempt to develop 

 as a whole, however limited their capacity to do so. 

 b. With respect to the second point, a priori, there would 

 appear no grounds for supposing that the effect of the 

 initiixl disturbance would be limited to the embryonal 



