MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 407 



plate, while others (Y. C. Y. O. 1 ) have wandered far and wide through the yolk. The embryo is 

 raised considerably above the general surface of the egg and the shell more closely invests 

 the egg than shown in the drawing. The embryonic membrane is not represented. 



Fig. 125 is a nearly median longitudinal section, and shows the relations of the thoracic-alt 

 dominal fold, the oesophagus, and the ventral or sternal surface between them. The loose and 

 irregular arrangement of cells immediately below the surface is most marked, and also the granu- 

 lar nature of the nuclei which is such a constant character. Numerous degenerating cells (S. S. C.) 

 are seen near the oesophagus, and amu-bilbrm cells can be traced from the thoracic abdominal fold 

 to the surface immediately behind it. 



The structure of this embryo is illustrated more completely by a series of transverse sections 

 (Figs. 114-124), the first two of which (PI. XLI, Figs. 114. 115) traverse the optic lobes, and the 

 third cuts the brain. The central mass of large cells which was noticed in the optic disks can no 

 longer be distinguished. The lobe (O. L.) is composed of similar cells with granular nuclei, the 

 superficial tier being somewhat the larger and columnar. In the brain the central cells are smaller 

 and stain more intensely than those at the surface (Fig. 116, S. O. G.). Wandering cells (Y. C.) 

 can be traced in their passage from the yolk to the optic lobes, brain, ventral nervous plate, and 

 other parts of the embryo. Between the lateral halves of the brain there is a shallow median fur- 

 row (Fig. 110, M. F.). This is continued backward into the much broader and deeper depression in 

 which the convex ventral side of the abdomen fits (PI. XLII, M. P.). The three following .sections 

 (Figs. 117-1 19) .pass through the (esophagus, and the ventral nerve thickening immediately behind it. 



About the oesophagus (Fig. 117, Std.) numerous chromatin balls (S., S. C.) are seen in the yolk, 

 and a mass of cells (Mes.) is met with at the base of the appendage and within its fold. These 

 elements are derived from the wandering cells and must be regarded as mesoblast. The fold of the 

 appendage consists essentially of a single layer of cells. Those elements which enter it undoubt- 

 edly go to form the musculature of the limb while the cells of somewhat similar appearance, which 

 are derived from the ectoblast, represent the ganglia and nerves. Ou either side of the oesophagus 

 the yolk has undergone important physical and chemical changes. The yolk spheres or blocks 

 are full of vacuoles and have a corroded and granular appearance, while in contact with the 

 embryonic cells there is a residue of small retractive granules. These vary considerably in size, 

 and some of them stain lightly in bsematoxylin and represent the last stages in the degeneration 

 of chromatin. The eroded and altered yolk (A. Y. S.) is represented in many of the sections. 

 Between the (esophagus and bases of the antenna 1 the yolk is absorbed, leaving a protoplasmic 

 reticulum (Fig. 117, Ret.). 



In Fig. 118 the mass of cells representing the tnandibular ganglion (Md. G.) is sectioned, and 

 in the following figure, the mandibles themselves (Fig. 119, Md.). Numerous cells, both in this 

 and the following sections, are seen in the course of their migration from the yolk to join the 

 ventral nerve-thickening. The latter, which is the rudiment of the nervous system, is at this 

 .stage scarcely thickened at all, on the middle line, below the level of the mandibles. Thus in Fig. 

 121 the buds of the first maxilla; (Mx. I.) are united by the primitive layer of epiblast. To this a 

 single migrating cell has attached itself on the middle lino. Migrating mesoblast cells (Mes.) also 

 pass into the fold of the appendage, and others (Fig. 120, Mes.) take up a position against the 

 epiblast of the body wall. The nuclei of the latter stain intensely and become flat, spindle shape 

 in section, and probably represent the nuclei of muscle cells. 



The structure of the abdomen at this stage is shown in Figs. 120-123. The body wall of the 

 dorsal side, consists of a single layer of columnar epiblast, while the ventral wall is thickened. 

 The hind gut (Fig. 122, Hg.) is a tube, the wall of which consists of a single layer of cells. It is 

 laterally compressed so that the lumen is hardly appreciable. The intervening cells (Mu.) largely 

 represent the rudimentary flexor and extensor muscles of the abdomen. A comparison of Figs. 

 12:; and 125 shows that cells extend from the thoracic-abdominal fold on all sides into the yolk. 



The cells at the surface in Fig. 124 have come mainly from the yolk (H.) and are in the posi- 

 tion where the heart is subsequently developed. Cells approaching the surface in this region are 

 very clearly shown in Fig. 125 (Y. C., Y. C. 1 ), which is a section through a somewhat younger 

 embryo. 



The wandering cells, as we have seen, abound in the parts of the yolk nearest the embryo. The 



