EARLY STAGES OF DEVELOPMENT OF THE MOUSE. 67 



the one point where the outer layer was incomplete a blasto- 

 pore. By the continued enlargement of the cavity which 

 appeared between the outer layer and inner mass the 

 blastodermic vesicle was formed, in which the inner mass of 

 rounded cells was attached at one point, the site of the 

 blastopore, to the inner surface of the hollow sphere of 

 flattened cells derived from the outer layer. The innermost 

 cells of the inner mass now began to spread out and grow 

 round the inner surface of this sphere, the remainder forming 

 a definite layer, the mesoderm, between the two primary 

 ones. The embryonic area was thus marked out, and it 

 became possible to definitely compare such a blastodermic 

 vesicle with the blastoderm of a chick, especially when it 

 was shown that the formation of the primitive streak, 

 amnion, and allantois was essentially the same in the two 

 cases. However, Rauber published almost at the same time 

 an account of his own independent researches, in which he 

 energetically maintained that what van Beneden had de- 

 scribed as mesoderm was in reality the embryonic ectoderm 

 (which was therefore derived from the inner mass), while 

 the flattened cells of the outer layer over these (since known 

 as Rauber's cells) disappeared, the remainder of the outer 

 layer fusing with the edges of the disc of embryonic ectoderm. 

 These conclusions van Beneden refused to accept until 

 compelled to do so by Kolliker's careful reinvestigation of 

 the subject published in 1882. Since then the researches of 

 Selenka on Mus, Arvicola, Cavia, and Pteropus, of Heape 

 and Liebei'kiihn on Talpa, of Hubrecht on Erinaceus, Sorex, 

 Tupaia, and Tarsius, and of Assheton on Sus and Ovis, have 

 all tended to show, firstly, that in the typical mammalian 

 blastocyst the embryonic epiblast, and frequently the true 

 amnion, as well as the whole of the hypoblast, are derived 

 from the inner mass, while the outer layer, which we may 

 now, with Hubrecht, speak of as the trophoblast, gives rise 

 to the false amnion, and comes into close connection with the 

 uterus, playing a very important part in the formation of the 

 placenta; and secondly, that while the mode of formation of 



