MEMOIllS OF THE NATIONAL ACADEMY OF SCIENCES. 409 



Waiuloring cells are now scattered tliroughout the entire eprg. They occur in abiiiidiince both 

 in proxiinily to tiie embryo itro[)er iuul on the sides of the body walls, and esjiecially in the region 

 immediately behind the thoracic-abdominal fohl. 



l''i{j. I.jI is a. median longitinlinal section tlirongh an embryo liki^ that shown in Fig. 130. The 

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

 si)herical, elongate, or wedge-ahajied. They divide in both planes, but most commonly in the 

 )>lanc i)erpcndicnlar to the surface. When we compare this section with the similar one (Fig. 125) of 

 the prece<ling stage the most striking difierence 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 tiattened and si)indleshaped. 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 laj'er 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 [)lanes, 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 dividingcells. Some of the cells next 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 retinogeu, since from it, or from a layer corre- 

 sponilingto it, the visual ap|)aratus of the eye is developed, while from the deeper layer or (/aM<;/io(;cH 

 the oi)tic ganglia of the eyestalk are formed. 



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

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

 and more compact, and .some of the cells next the 5 oik 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 to represent a ruilimentary perineurium, but, as a well developed covering 

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

 Fig. 13-1 corresponds closely with Fig. 117. It shows the section of the (esophagus and of the 

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

 at the ba.se of the ap]iendage) 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 

 a'Soi)hagus (Sfd.) is suspended to the body walls by rudimentary muscles (Mn.), the cells of whicli 

 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 a'sophagiis. 

 Fig. i;{7, which is fiom a stage intermediate between the two just considered, gives addiiional 

 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 ba.se of the appendage can also be 

 distinguished. 



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

 Here we recognize a tier or i)late 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 

 (I'^ig. 135, A. Y. S.) is ab.sorbed or converted into a granular residue. This layer represents the 

 enUiUast or the epithelium of the mesenteron already described. Numerous wandering cells are 

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

 closely asso(;iated with the epithelium of the hiiulgut. They unite the mesenteron to the hindgut, 

 and it is imjwssible to say exactly where the one begins and the other ends. Between this ento. 

 blastic j)latc and the surface e))iblast (Ect.) numerous <'ells are interpolated (l-'igs. 133, 1.35, Mes.), 

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



