204 BULLETIN OF THE 



A. liana. 



Stage I. 



Plate I. Figs. 1-3. 



At the first stage which I shall describe the embryo has departed only 

 a little from the spheroidal form presented by the egg during segmenta- 

 tion. The medullary plate is widely open, its lateral margins being only 

 slightly elevated above the general surface. At the hinder end of the 

 medullary plate the blastopore is plainly visible. An idea of the exter- 

 nal form of the embryo can be gained by reference to Goette's figure of 

 Bombinator ('7.1, Taf. III. Fig. 41), or to van Bambeke's of the Axolo- 

 tyl ('80, PL XII. Fig. 9). In water of 15 to 18° C. eggs of R. sylvatica 

 reached this stage in about sixty hours after fertilization ; the eggs of 

 "R. halecina develop somewhat more slowly. 



The general relations of the germinal layers at this stage are shown in 

 Figure 2. The ectoderm consists of two distinct layers (Figs. 3 and 7, 

 ec'drm.' and ec'drm."). Except in the region of the medullary thicken- 

 ing (la. med.), which is produced by a proliferation of the deeper of 

 tli^se two layers, the ectoderm is nearly uniform in thickness. The two 

 layers present slightly different histological characters. In the outer 

 layer (Figs. 3 and 7, ec'drm.') the cells are large and columnar, and their 

 external surfaces project as rounded eminences, giving a roughly granular 

 appearance to the surface of the embryo. Each cell contains scattered 

 pigment grannies, which are especially massed along its external face. 

 Small yolk spherules (spk. vt.) are present in considerable numbers. 

 The cells of the deep layer (ec'drm.") are smaller than those of the outer, 

 and are somewhat flattened. The pigment granules are scattered through- 

 out the cells of this layer, without showing special accumulations. The 

 yolk spherules present the same appearance as those of the superficial 

 layer. 



The entoderm and yolk cells (Fig. 2, en'drm. and cl. vt.) form the great 

 mass of the interior of the embryo. The wide lumen of the gut trav- 

 erses the dorsal portion of this mass. The chorda (n'ed.) has the form 

 of a longitudinal ridge, imperfectly cut off from the entoderm below, and 

 in contact with the medullary plate above. A single cell layer (en'drm.) 

 on each side of the chorda forms the dorsal roof of the intestine. As 

 this layer passes out laterally, it increases in thickness, becomes several 

 cells deep, and finally merges in the mass of large yolk cells (cl. vt.) lying 

 ventral to the intestinal cavity. All the cells of the entoderm contain 

 large yolk spherules. Pigment is present in considerable quantity in the 



