108 J. Thos. Patterson. 



to be regarded as growing forward. He, however, not having studied 

 the earlier stages, naturally supposed that the entoderm was an 

 outgrowth from the posterior germ-wall, and thus missed the key to 

 the origin of this germ-layer. 



During the course of further development the entoderm com- 

 pletely penetrates the subgerininal cavity (Fig. XV, SG), and at the 

 same time the mass of cells (lower cells of the dorsal lip) at its 

 posterior border thins out to a single layer, thus showing that these 

 cells contribute to the entoderm in its forward growth. 



IV. Experimental Studies. 



While many of the foregoing conclusions were first deduced from 

 data gathered in a study of sections, yet they are of such a nature 

 that experimental tests can be applied readily. Only a few of the 

 many experiments that have been performed can be offered at this 

 time, and these are selected, not because they are of any more interest 

 than the others, but rather because they throw light on that mooted 

 question, "How does the vertebrate embryo arise ?" The two views 

 that have been held by students of vertebrate embryology in regard 

 to this question are too well known to need any discussion here. 

 Both theories have been defended by able workers, but too often the 

 attempt has been made to support the one to the exclusion of the 

 other. This has been especially true of those who hold to the theory 

 of differentiation. 



The results obtained by experimental investigators have not been 

 uniform. In the main, writers have been willing to admit that only 

 a modified form of concrescence is found in the formation of the 

 embryo. In the few desultory experiments (Assheton, '96, Kopsch, 

 '02) that have been made on the chick blastoderm only negative 

 results have been found. This failure to secure positive evidence 

 is due to two causes. In the first place, the technique has not been 

 sufficiently refined. Thus, Assheton used sable hairs which he in- 

 serted in the unincubated blastoderm on either side of the axial 

 line on the boundary between the areas opaca and pellucida. The 

 results were negative, as one might expect; for who would suppose 

 that the force exerted by the movement of materials in the delicate 



