MEMOIRS OF TIIK NATIONAL ACADP]MY OF SCIENCES. 407 



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

 raist'd considerably above the general surface of the egg and tl»o shell more closely invests 

 the t'gg thiin shown in the drawing. The embryonic membrane is not represented. 



l''ig. 125 is a nearly median longitudinal section, and shows the relations of the thoracic-ab- 

 donunal fold, the o'sophagus, and the ventral or sternal surface between them. The loose and 

 irregular arrangement of cells immediately below the surface is most marked, and al.so the granu- 

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

 are seen near the u'sophagus, and anuebiform cells can be traced from the tlioracic abdominal fold 

 to the surface immediately behind it. 



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

 (Figs. 114-124), the first two of which (IM. Xi.i, Figs. 114, 11.5) traverse the optic lobes, and tlie 

 third cats the brain. The central mass of large cells which was noticed iu the optjc disks can no 

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

 superlicial tier being somewhat the larger and columnar. In the brain the central cells are smalU>r 

 anil stain more intensely than those at the surface (Fig. 110, S. O. G.). Wandering cells (Y. C.) 

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

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

 row (Fig. 116, M. F.). This is continued backward into the much broader and deeper depression iu 

 which the convex ventral side of the abdomen fits (IM. xlii, M. F.). Tiie three following .sections 

 (Figs. 1 17-119) jiass tlirough tliefflSophagus,and the ventral nerve thickening immediately belli ml it. 



About the tesophagus (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 ai)pendage and within its fold. These 

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

 appendage (ionsists essentially of a single layer of cells. Tlio.se elements whi(!h 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. On either side of the (esophagus 

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

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

 embryonic cells there is a residue of small refractive granules. The.se vary considerably in size, 

 and some of them stain lightly in ha'matoxylin and represent the last stages in the degeneration 

 of chromatin. The eroded and altered yolk (A. Y. S.) is rciiresented in many of the; section.s. 

 Between the lesophagus and bivses of the aiitenu* the yolk is absorbed, leaving a protoplasmic 

 reticulum (Fig. 117, Ret.). 



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

 in the following figure, the mandibles themselves (Fig. 110, Md.). Numerous cells, both in this 

 and the following sections, are seen in the course of their migration frcun 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 maxillae (Mx. 1.) are unite<l by the primitive layer of epibla.st. To this a 

 single migrating cell has attached itself on the middle line. Migrating me.soblast cells (Mes.) also 

 pass into the fold of the ajjpendage, 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, s])indle shape 

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



The structure of the abdomen at this stage is shown in Figs. ]20-12.'>. 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 con.sists of a single layer of cells. It is 

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

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

 12;{ and 12.5 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 (11.) and are in the i)osi- 

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

 very clearlj' shown in Fig. 125 (Y. C, Y. C), 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 



