WEYSSE. — BLASTODERMIC VESICLE OF SUS SCROFA. 289 



attenuated at their regions of contact, appearing spindle-shaped in 

 section, the nucleus occupying the swollen portion, so that the dividing 

 walls are apparent in the region of the germinal disk only, where the 

 axes of the cell are nearly equal. In my oldest embryos there is no 

 sio-n of the formation of mesoderm, or of the medullary groove. With 

 this description of the embryonic vesicle in general, I now proceed to 

 the consideration of the more detailed structure of several embryos 

 which I have selected for illustration here. I have chosen these because 

 they show in a typical manner the phenomena which all present, and 

 with drawings from them I can explain intelligibly any variations from 

 the type. 



2. Detailed Account of Observations on the Germinal Dish 

 First Stage. (Plate II Figs. 7, 8, and 9.) 



The first embryo which I shall describe is represented by drawings 

 (Plate II. Figs. 7, 8, and 9) of three sections of the germinal disk. 

 This embryo was taken from a sow killed ten days after coitus, which 

 contained eleven embryos. The embryo in question was with one 

 exception the smallest found, but it should be noted here that the ratio 

 between the size of the embryo and that of the germinal disk is not 

 constant for different embryos ; and furthermore, that the ratio of the 

 size of the disk to its degree of development varies; e. g. Figs. 11, 12, 

 and 13, Plate II., were drawn from an embryo much larger than that 

 from which Figs. 7, 8, and 9 were made ; the two embryos came 

 from the same uterus, and yet the former disk is slightly smaller 

 than the latter ; on the other hand, it represents a much later stage of 

 development, as I shall show hereafter, and this fact is doubtless suffi- 

 cient to account for the difference, in size. The whole vesicle from 

 which Figs. 7, 8, and 9 were drawn measured about 1.25 mm. in 

 diameter. As I have already said, the vesicles are flattened out and 

 greatly wrinkled, so that such measurements are at best approximate 

 only, though they serve to give a general idea of the relative sizes 

 of the embryos. The germinal disk, which lies on a part of the vesicle 

 that is not folded, is slightly elliptical in outline, the chief and trans- 

 verse axes being about 0.11 mm. and 0.167 mm. long respectively; 

 the sections were made nearly at right angles to the shorter axis, 

 which, as I think I can show later, represents the chief axis of the 

 future embryo. 



The ectoderm consists of the characteristic large cells with spherical 

 nuclei already described. The entodermal cells form a complete layer 

 VOL, XXX. (n. s. xxii.) 19 



