206 BULLETIN OF THE 



the mesoderm exhibits, particularly such as occur in the region where 

 the pronephros is subsequently developed. For this purpose I have 

 selected two embryos of Stage I. which show slightly different condi- 

 tions. The account will first relate to the specimen which is shown, by 

 the less differentiation of the medullary plate as well as by other features, 

 to be the younger. This embryo measures 2.31 mm. in length. In fol- 

 lowing a series of cross sections forwards, the three germ layers become 

 apparent at about 0.35 mm. from the posterior end, or a short distance 

 in front of the blastopore. Here the structure of the mesoderm is rather 

 obscure, since in a transverse section of the animal this layer is cut 

 obliquely. The condition, however, is here nearly the same as that 

 which I am about to describe for a more anterior section. 



Figure 3 represents a section of this embryo 0.91 mm. from the pos- 

 terior end. On the ventral side of the embryo the mesoderm consists of 

 two layers, each of which is only a single cell in thickness. These two 

 layers, which represent somatopleure and splanchnopleure, are separated 

 by a narrow space, the ccelom (coeL). In the lower left-hand corner 

 of the figure, the beginning of this two-layered condition of the meso- 

 derm can be seen. On following the mesoderm towards the dorsum, it 

 becomes gradually thicker. In the mesoderm of this region there is 

 found an extensive cavity (coel.), which is usually irregular in outline, 

 and might be mistaken for a wholly artificial condition. That the two 

 layers were once in contact is shown by the correspondence of outline on 

 the two sides of the space. The separation along this line is so regular, 

 however, in successive sections, and recurs so frequently in other em- 

 bryos, that the cavity must be regarded as an artificial expansion of an 

 already existing split, rather than as an indifferent rupture of a solid 

 mass of cells. In many sections of this embryo it is easy to trace a line 

 of division reaching from the ventral cavity (ccelom) to the large lateral 

 cavity just described. This, then, represents a portion of the coelom 

 (normally, I believe, closed), and the layers of mesoderm on the two 

 sides of it are consequently somatopleure and splanchnopleure. The 

 mesoderm in this region, as I have stated, is several cells deep. Along 

 the inner and outer edges of the wedge-shaped plate of tissue constitut- 

 ing the mesoderm of either side, the cells, except where artificial rup- 

 tures occur, are in close contact, and form an epithelial lamella. The 

 central portion of the plate, where this is more than two cells in thick- 

 ness, contains cells of a more rounded shape, which do not form definite 

 rows, but which are closely applied to the outer layer, — a condition 

 which becomes quite evident when the coelom is artificially enlarged. 



