252 ATERS ON THE DEVELOPMENT 



the neck. The cells are bent in various ways depending upon the relations of their nuclei to 

 the wall of the pad. The only larval organs which in any way resemble these are, so far 

 as I am at present informed, the peculiar mushroom-shaped bodies described by Rathke 

 (38, pp. 27-32, Taf. 2, figs. 1-5) for Gryllotalpa. The author considered them to be 

 respiratory in function but he was not able to establish his interpretation. 



The central nervous system, which made its appearance early in the development of the 

 embryo, is still united with the ectoderm at the time of revolution, but with the comple- 

 tion of this act, which seems to add a new impulse to the development of the entire 

 organism, the ganglionated cords that form the nervous system of the thoracic and abdom- 

 inal regions become cut off from the superficial ectoderm and lie free within the body 

 cavity. The median cord, which was formed by a modified invagination of the superficial 

 layer of the ectoderm, immediately fuses with the lateral cords along its dorsal and lateral 

 surfaces, but its ventral surface still forms a portion of the ventral surface of the body of 

 the embryo. By the overgrowth of the superficial cells on either side of this cord it 

 becomes inclosed within the body. These three parts of the nervous system now form a 

 single nodulated rod which lias no structural connections with the rest of the body, nor with 

 the brain. Since the enlargements of the two lateral cords occur at regular intervals and 

 opposite each other, each segment of the young embryo is furnished with two ganglia, but 

 when the median ingrowth passes between and fuses with them, they become structurally 

 connected by two bundles of fine transverse filaments which arise out of the substance of 

 this invaginated part. These bundles of fibres do not remain distinct but soon fuse, after 

 which they are seen to connect the central portions of the ganglia. Between the succes- 

 sive pairs of ganglia the median ingrowth atrophies, -and at the time of the closure of the 

 dorsal wall of the body there is seen between the connecting cords of two adjacent pairs of 

 ganglia, a small triangular or cylindrical mass of cells, concerning the fate of which 

 I am not absolutely certain. I believe, however, that they go to form a part of the inter- 

 nal skeleton. The chitinous rods in the thoracic region to which the muscles of the legs 

 and wings are attached probably arise from the remnants of this median invagination, but 

 in the abdominal region they may disappear entirely without giving rise to such structures. 



The two lateral ganglia of each segment ultimately become fused into one, so that on 

 dissecting out the nervous system of the embryo represented in pi. 19, fig. 4, all the gan- 

 glia presented the appearance represented in pi. 10, fig. 0. They are ovoid bodies con- 

 nected by two longitudinal commissures which at this stage are so short as to leave the 

 ganglia in contact with one another; later these long commissures increase much in length 

 and the ganglia thus become widely separated. Before hatching the three ganglia of the 

 maxillary region (i. e. the primitive pairs of ganglia supplying the mandibles, the first and 

 the second maxillae) fuse into one mass 1 — the suboesophageal ganglion. This nervous centre 

 is the largest in the body with the exception of the brain, which it nearly equals 

 in size. In pi. 21, fig. 39, is figured a section through this ganglion before the 

 fusion of its fibrous portions. The fibres of the longitudinal commissure are similar to 



1 Grabcr (21, page 428), says that in insects the ganglia of does not affirm, however, that he has himself observed this 



the mandibular segment become the commissural cords, to be the case. It is improbable that there exists in differ- 



while the suboesophageal ganglion is composed of the prim- ent insects such a difference in the manner of the formation 



itivc ganglia of the first and second maxillary segments. He of the commissural cords and the suboesophageal ganglion. 



