MEMOIES OF T1IK NATIONAL ACADEMY OF SCIENCES. 409 



Wandering cells are now scattered throughout the entire egg. They occur in abundance both 

 in proximity to the embryo proper and on the sides of the body walls, and especially in the region 

 immediately behind the thoracic-abdominal fold. 



Fig. 131 is a median longitudinal section through an embryo like that shown in Fig. 130. ' The 

 outer or superficial cells are generally columnar and have distinct boundaries. Their nuclei are 

 spherical, elongate, or wedge-shaped. They divide in both planes, but most commonly in the 

 plane perpendicular to the surface. When we compare this section with the. similar one (Fig. 125) of 

 the preceding stage the most striking difference is the great increase in the length of the thoracic- 

 abdominal fold and the ventral thickening of the nervous system. Some of the cells of the latter 

 next the yolk (Mes.) are flattened and spindle-shaped. They have recently come from the yolk 

 and joined the nervous system, and it is possible that these and similar cells represent primitive 

 connective tissue envelopes. 



In a more lateral section (Fig. 129) the optic lobe is seen to consist of two portions, a super- 

 ficial layer of large cells (O. E.) and a deeper layer (G. L.). The first is continuous with the 

 general epiblast over the surface of the body. Its cells multiply in both planes, and some of them 

 pass below to join the deeper layer. The cells of the latter multiply by the usual mitosis and also 

 receive accessions from the yolk. The plane of section passes through the equatorial plate of one 

 of these dividing cells. Some of the cells nest the yolk (Mes.) are flattened like those just described 

 in the ventral nervous thickening, but this condition appears to be somewhat transitory. The 

 outer layer of the optic lobe may be regarded as a retinogen, since from it, or from a layer corre- 

 sponding to it, the visual apparatus of the eye is developed, while from the deeper layer or gangliogen 

 the optic ganglia of the eyestalk are formed. 



A comparison of the transverse sections (Figs. 128, 132-135) with corresponding sections of 

 the previous stage (Figs. 115, 117. 121, 124) shows some interesting changes. The brain is larger 

 and more compact, and some of the cells next the yolk are flattened (Fig. 132, Mes.) and bear a 

 resemblance to muscle or connective tissue cells. They originate from the cells marked Ct. S. in 

 Fig. 110, and come from the yolk. Like the cells already mentioned in the optic lobes and ventral 

 nervous system, they seem so represent a rudimentary perineurium,but, as a well developed covering 

 of the nervous system is not present until a considerably later stage, they are probably transitory. 

 Fig. 134 corresponds closely with Fig. 117. It shows the section of the oesophagus and of the 

 ganglia of the second antennae. In the younger stage the ganglion (seen to the left in Fig. 117, 

 at the base of the appendage) is a small, loose mass of cells associated with the surface epiblast, 

 while in the older embryo it is in contact with the wall of the oesophagus, is more compact, and its 

 cells are somewhat differentiated. The central nuclei are smaller and stain most intensely. The 

 esophagus (Std.) is suspended to the body walls by rudimentary muscles (Mu.), the cells of which 

 are much elongated. They are derived from migrating mesoblastic cells like those seen in the 

 earlier stage (Fig. 117, S. C.), or like the one shown in this figure to the left of the (esophagus. 

 Fig. 137, which is from a stage intermediate between the two just considered, gives additional 

 evidence of this role of the wandering cells. Fig. 128 furnishes a very interesting comparison with 

 Fig. 121. In the latter, cells abound in the yolk adjoining the rudiment of the ventral nervous 

 system, which is represented by the primitive epiblast on the middle line. In the older stage 

 scattered mesoblast cells are greatly reduced in number and the ventral thickening is very 

 marked. Cells of recent derivation from the yolk (Mes.) at the base of the appendage can also be 

 distinguished. 



In Fig. 13J, as in Fig. 124, the plane of section is just behind the thoracic-abdominal fold. 

 Here we recognize a tier or plate of tall, columnar cells (End.), the nuclei of which lie at the deeper 

 ends of the cells or on the side away from the yolk. In the presence of these bodies the food yolk 

 (Fig. 135, A. . S.) is absorbed or converted into a granular residue. This layer represents the 

 I'titobhifit or the epithelium of the mesenteron already described. Numerous wandering cells are 

 encountered (Figs. 124, Mes.; 135, Y. C.), which take up a peripheral position, and from the first are 

 closely associated with the epithelium of the hindgut. They unite the mesenteron to the hiudgut, 

 and it is impossible to say exactly where the one begins and the other ends. Between this ente- 

 blastic plate and the surface epiblast (Ect.) numerous cells are interpolated (Figs. 133, 135, Mes.), 

 which are undoubted mesoblast. They are directly continuous with the layer of mesoblast (Fig. 



