EXPLANATION OF PLATES XIII, XIV & XIY— Continued. 

 List of Reference Letters — continued. 



ca V. Cardinal vein. 

 mp. Muscle-plate. 

 m p'. Early formed mass of muscles. 



ov. Oviduct. 

 p ov. Projection which becomes the ovary. 

 w d. Wolffian duct. 

 p wd. Primary points of involution from the pleuro-peritoneal 

 cavity by the coalescence of which the Wolffian duct is 

 formed. 

 su.r. Supra renal body. 

 p n. Pineal gland. 

 ht. Heart. 

 V. Blood-vessel. 



All the figures were drawn with the Camera Lucida. 



Fig. 1. — Section parallel with the long axis of the embryo through a 

 blastoderm, in which the floor of the segmentation cavity {s c) is not yet 

 completely lined by cells. The roof of the segmentation cavity is broken. 

 (Magnified 60 diam.) The section is intended chiefly to illustrate the dis- 

 tribution of nuclei {n) in the yolk uuder the blastoderm. One of the chief 

 points to be noticed in their distribution is the fact that they Term almost 

 a complete layer under the floor of the segmentation cavity. This probably 

 indicates that the cells whose nuclei they become take some share in 

 forming the layer of cells which subsequently {vide fig, 4) forms the floor of 

 the cavity. 



Fig. 2. — Small portion of blastoderm and subjacent yolk of an embryo at 

 the time of the first appearance of the medullary groove. (Magnified 300 

 diam.) 



The specimen is taken from a portion of the blastoderm which will form 

 part of the embryo. It shows two large nuclei of the yolk {n) and the net- 

 work in the yolk between them ; this network is seen to be closer around 

 the nuclei than in the intervening space. The specimen further shows that 

 there are no areas representing cells around the nuclei. 



Fig. 3. — Section parallel with the long axis of the embryo through a 

 blastoderm, in which the floor of the segmentation cavity is not yet covered 

 by a complete layer of cells. (Magnified 60 diam.) 



It illustrates (1) the characters of the epiblast, (2) the embryonic swell- 

 ing (e s), (3) the segmentation cavity {s e). It should have been drawn 

 upon the same scale as fig. 4; the line above it represents its true length 

 upon this scale. 



Fig. 4. — Longitudinal section through a blastoderm at the time of the 

 first appearance of the embryonic rim, and before the formation of the me- 

 dullary groove. (Magnified 45 diam.) 



It illustrates (1) the embryonic rim, (2) the continuity of epiblast and 

 hypoblast at edge of this, (3) the continual differentiation of the lower 

 layer cells, to form, on the one hand, the hypoblast, which is continuous with 

 the epiblast, and on the other the mesoblast, between this and the epiblast; 

 (4) the segmentation cavity, whose floor of cells is now completed. 



N.B. — The cells at the embryonic end of the blastoderm have been made 

 rather too large. 



Fig. 5.— Surface view of the blastoderm shortly after the appearance 

 of the medullary groove. To show the relation of the embryo to the 

 blastoderm. 



