184 H. V. WILSON AND BLACKWELL MARKHAM 



monsters, in which the blastoderm does not grow round the 

 yolk, but remains small, while an embryonic body is developed 

 extending completely through it from posterior to anterior edge 

 (Kopsch, '04, .p. 95, Taf. X, Fig. 118). 



If the organization of the lateral blastopore lip is an unusual, 

 abnormal process, spina-bifida embryos become no less interest- 

 ing than they have been hitherto, since they tell us plainly that 

 here is tissue which may be activated to develop, in very atypical 

 fashion, into certain of the specific structures. From this point 

 of view, such embryos may be thought of as arising in the follow- 

 ing fashion. The axial body which normally forms in front of 

 the blastopore lip is short. It grows in length especially at the 

 posterior end which is carried backward, the growth being in 

 part due to the incorporation of tissue belonging to the blasto- 

 pore lip. The indifferent mass of tissue at the posterior end of 

 this embryonic body is constantly being organized and added to 

 the organs in front of it, some to neural tube, some to notochord, 

 etc. Frankly abandoning deterministic theories which would 

 see in the tail bud of a vertebrate embryo not indifferent tissue, 

 but neural, notochordal, and other kinds of cells, we ask what 

 brings it about that certain cells go to one, other cells to another 

 organ? Plainly O. Hertwig and the thinkers of that school are 

 right; it is position that determines the fate of the cells. Those 

 behind the notochord become notochord. Those behind the 

 neural tube become neural tube. What underlies this phenom- 

 enon? The answer seems clear: the already differentiated or- 

 gan, notochord, e.g., exerts a controlling influence on the con- 

 tiguous, indifferent cells behind it and makes them into its like.^ 



Now when the backward growth of the blastopore lip (to be 

 construed as part of the general closure of the blastopore), and 

 hence of the axial embryonic body, is prevented, what is the sit- 



1 Roux has discussed this kind of influence ("cine eigenthtimliche ordnende 

 und gestaltende Wirkung," 1888, p. 505 and passim) in the case of half-embryos 

 of the frog produced by killing, or nearly killing, one of the first two blastomeres. 

 In the formation, 'postgeneration,' of germ layers in the operated half, the or- 

 ganizing influence extends outward from the edge of each, already formed, germ 

 layer of the normal half. This progressive differentiation of relatively indiffer- 

 ent stationary material, where the "differentiative stimulus passes from the al- 



