I Xl'LANATION OF PLATES 3 AND 4. 



LANATION OF PLATES 3 AND 4. (V. p. 60.) 



COMPLETE LIST OF REFERENCE LETTERS. 



a I. Alimentary canal, a o. Dorsal aorta, au v. Auditory vesicle, b d. Forma- 

 tive cell probably derived from the yolk, ca v. Cardinal vein, c h. Notochord. 

 c h'. Thickening of hypoblast to form the notochbrd. e b. Line indicating the edge 

 of the blastoderm, e p. Epiblast. */. Epidermis, e r. Embryonic rim. e s. Em- 

 bryonic swelling, gl. Glosso-pharyngeal nerve, h. Head. ht. Heart, hy. Hypo- 

 blast. //. Lower layer cells. / y. Line of separation between the blastoderm and 

 the yolk. m. Mesoblast. me. Medullary canal, m g. Medullary groove, m p. 

 Muscle-plate, m /. Early formed mass of muscles, n. Peculiar nuclei formed in 

 the yolk. n'. Similar nuclei in the cells of the blastoderm. n a. Cells which help 

 to close in the alimentary canal, and which are derived from the yolk. y. Network 

 of lines present in the food-yolk, o I. Olfactory pit. op. Eye. ov. Oviduct, p n. 

 Pineal gland, p ov. Projection which becomes the ovary. / p. Pleuro-peritoneal 

 cavity. //'. Remains of pleuro- peritoneal cavity in the head, pr v. Protovertebrae. 

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

 cence of which the Wolffian duct is formed, s g. Segmentation cavity, s o. Somato- 

 pleure. so s. Stalk connecting embryo with yolk-sac, s p. Splanchnopleure. sp n. 

 Spinal nerve, su r. Supra-renal body, t s. Caudal lobes. z>. Blood-vessel, v g. 

 Vagus nerve, v. Fifth nerve, vn. Seventh nerve, v c, i, 2, 3, &c. ist, 2nd and 

 3rd &c. visceral clefts, v p. Vertebral plates, w d. Wolffian duct. x. Peculiar body 

 underlying the notochord derived from the hypoblast. y k. Yolk spherules. 



All the figures were drawn with the Camera Lucida. 



PLATE 3. 



Fig. i. Section parallel with the long axis of the embryo through a blastoderm, 

 in which the floor of the segmentation cavity (s g) 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 distribution of nuclei (n) in the yolk under the blasto- 

 derm. One of the chief points to be noticed in their distribution is the fact that they 

 form almost a complete layer under the floor of the segmentation cavity. This pro- 

 bably 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. . 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 shews two large nuclei of the yolk (n) and the network in the yolk 

 between them ; this network is seen to be closer around the nuclei than in the inter- 

 vening space. The specimen further shews that there are no areas representing cells 

 around the nuclei. 



.,. 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.) 



