242 Prof. W. Salenskj on the 



stages of development gives rise to tlie atrial tubes and the 

 nerve-ganglion. The bulk of the embrjonic mass consists 

 of the undifferentiated elements of the two other germinal 

 layers, and may therefore be termed the meso-endoderm ; 

 before this gives rise to the rudiments of various organs it 

 undergoes a differentiation, resulting in its splitting into two 

 germinal layers, the mesoderm and endoderm. The differen- 

 tiation of the endoderm occurs tolerably late — not until after 

 the formation of the coelomic cavities in the mesoderm. The 

 formation of the coelomic spaces and their metamorphoses are 

 wliat we now have to consider. 



If we examine sections from the nuclear mass, in 

 which mesoderm and endoderm are represented by a still 

 undifferentiated mass of cells, we at once notice in the interior 

 of these sections several lacuna-like cavities which as yet 

 have no connexion with one another. These cavities are the 

 earliest rudiments of the subsequent coelomic spaces, which 

 convert the solid mesoderm into two coelomic sacs. Whether 

 or not these cavities are symmetrically arranged from the 

 first I cannot decide with certainty, as I had at my disposal 

 but few embryos in these stages. At any rate they appear 

 to be symmetrically arranged in the following stage, in which 

 the nuclear mass flattens out and assumes the form of a 

 germinal disk. It is very probable that all the isolated 

 cavities coalesce at this period, since the coelom is now no 

 longer represented by several separate spaces, but by two 

 large cavities lying one on each side of the longitudinal axis 

 of the germinal disk. At much the same time as this 

 important changes also take place in the germinal disk itself; 

 the lower surface of the latter recedes from the upper surface 

 of the yolk, in consequence of which a space is left between 

 the yolk and the germinal disk, which is subsequently trans- 

 formed into the enteric cavity. The mutual relations of the 

 two spaces, the enteric cavity and the coelom, can be deter- 

 mined by means of sections ; and in successful ones we can 

 clearly see that the two coelomic sacs open into the cavity of 

 the intestine. In the median line between the two openings 

 of the coelom, in the axial portion of the germinal disk, there 

 projects into the enteric cavity a longitudinal ridge, which is 

 likewise traversed by a canal. The opening of this canal I 

 was not able to make out ; but with regard to the interpreta- 

 tion of the two lateral openings of the coelomic sacs, their 

 relation to the intestinal cavity points to the conclusion that 

 we have in these openings the homologues of those described 

 by van Beneden and Julin, through which the primitive 

 enteric cavity communicates with the coelomic sacs. Although 



