304 PROCEEDINGS OF THE AMERICAN ACADEMY. 



layers he believed to represent the ectoderm, mesoderm, and entoderm 

 respectively. 



Other investigators have found different conditions, hovrever, 

 which disprove this theory. I merely mention it ; first, because 

 my own embryos show clearly a marked outer layer, which I have 

 termed ectoderm, lined with a flattened layer, the entoderm, and 

 outside of all, unmistakable " Deckzellen," which would have to be 

 derived from the outermost layer of cells at a stage such as Van 

 Beueden described ; and, secondly, because not long ago Duval ('91) 

 found, as described in his work on the rat and the mouse, young 

 embryos to which he gave an interpretation very similar to that of 

 Van Beneden for the rabbit. He describes a hollow vesicle, consist- 

 ing of an outer layer of nearly isodiametric cells (see Duval, '91, Plate 

 I., Figs. 73 and 74), with a number of larger, somewhat irregular 

 cells inside, attached to the outer layer at one pole. He considers 

 these two sets of cells ectoderm and entoderm respectively. As 

 Duval describes the subsequent development, it is difficult to interpret 

 these otherwise ; but it should be remembered that Selenka ('83) 

 considei'ed the outer layer a " Deckschicht," and that Robinson ('92), 

 working on the same animals, has reached conclusions widely different 

 from those of Duval. I should like further to call attention to 

 Duval's Figs. 75 and 79, Plate I., which resemble strongly those of 

 other investigators on the rabbit, the mole, and the shrew, and which 

 would seem to represent a vesicle consisting of an outer layer of 

 somewhat flattened cells, and an inner mass differentiated into two 

 distinct regions, very much as Heape ('83, Plate XXIX. Fig. 20) 

 has shown it in the mole. 



The second theory is held by a larger number of investigators, 

 perhaps, than any other. It maintains that the flattened cells of the 

 outer layer become columnar and form the ectoderm of the extra- 

 germinal region. The inner mass of cells differentiates into two 

 superposed parts; the inner becomes the entoderm and comes to line 

 the inner surface of the ectoderm ; the cells of the outer part be- 

 come columnar, and, fusing with the cells of the outer layer, form the 

 ectoderm of the germinal disk. Balfour ('81), in conjunction with 

 Heape, thought he was able to trace the actual process of transforma- 

 tion of the flattened into columnar cells. Essentially the same views 

 are advocated by Rauber ('75), Lieberklihn ('79), Kolliker ('80), aid 

 Hubrecht ('90). 



My material supplies no evidence whatever of any transformation 

 of the outer layer of cells, or " Deckschicht,'' into true ectodermal 



