1904.] NATURAL SCIENCES OF PHILADELPHIA. 215 



In fig. 38, p\ the first cells of the embryonic pharynx are distin- 

 guished from the surrounding blastomeres by their different staining 

 qualities. Figs. 39-41 show the characteristic structure of such a 

 pharynx which is well developed, but not yet functional. Fig. 40 

 is a median vertical-section through- the pharynx, fig. 39 a median 

 cross-section through the central cells (h), and fig. 41 a cross-section 

 through the four inner cells (d). The cells which surround the lumen 

 of the pharynx are twelve in number — four somewhat flattened sur- 

 face cells (a), four cylindrical central cells (6), and four nearly spherical 

 inner cells (d). MetschnikofT ('83) suggests that the latter group of 

 four cells, supposed by some to represent the primary endoderm, may 

 serve as a valve to prevent the escape of yolk-cells. The central cells 

 are surrounded by a considerable number of smaller cells radially 

 arranged and supposed to be muscle-cells serving to open the pharjmx. 

 Figs. 39 and 40 also show wandering blastomeres in all parts of the 

 sjmcitial yolk-material of the embryo. A few of these are flattened 

 to form a partial epithelium. Fig. 42 is a section through a functional 

 pharynx taking in yolk-cells (y^). The central cells (6) are much flat- 

 tened to form the lining of the lumen, and the muscle-cells are length- 

 ened radially. The two inner cells shown in dotted outline belong 

 to the next section. 



Up to the time when the embryonic pharynx becomes functional, 

 the embryo is a solid ball of yolk in the form of a syncitium containing 

 scattered blastomeres, with the developing pharynx at one side, in 

 the region where segmentation began. Here and there over the sur- 

 face are flattened blastomeres forming an incomplete epithelium 

 (figs. 39-40). As the yolk is sucked in, the embryo becomes a hollow 

 ball filled with yolk-cells (figs. 43-47). [In these and the following 

 figures the space occupied by the yolk cells sucked in by the embryonic 

 pharynx — the secondary j^olk (i/^) — is not filled in.] Fig. 43 is a 

 section of a nearly spherical embryo from a capsule in which some yolk 

 still remained around the embryos. Fig. 44 is a cross-section of a 

 flattened embryo of full size, all the yolk outside of the embryos having 

 disappeared. In these sections the blastomeres (b) are scattered in 

 the primary yolk-material (y^) of the embryonic surface layer of the 

 embryo, and still possess the characteristics of the earlier blastomeres, 

 deeply-staining cytoplasm and large nucleus containing a conspicuous 

 nucleolus. Figs. 45 and 46 show parts of sections from somewhat 

 older embryos, where the embryonic pharynx (p^) is degenerating and 

 the blastomeres have multiplied so as to nearly fill the embryonic 

 layer, very little yolk remaining among them. The embryonic pharynx 



