496 COMPARATIVE ANATOMY CHAP. 



further dorsally. In D the fold (which proceeded from the amnion and a part 

 of the serous envelope) has bent back and the dorsal plate has contracted still 

 more. In E the folds have overgrown the dorsal plate and their edges have grown 

 together in the middle line. By this process the so-called dorsal tube comes into 

 existence, and then sinks into the yolk. The ectoderm then joins over the dorsal 

 side of the embryo. The tube of the mid-gut enclosing the yolk, together with the 

 dorsal tube, is formed by the complete surrounding of the yolk by the endoderm. 

 The dorsal tube is then broken up with the yolk and reabsorbed within the mid-gut. 

 In other insects these processes take the same course. The most important 

 difference is caused by the fact that from the first the yolk penetrates between the 

 amnion and the serosa, so that the embryo with the amnion seems to be imbedded 

 deep in the yolk. The amnion remains connected with the serosa at one place only. 

 At the place where the two membranes adhere a rent arises later, through which the 

 embryo together with the amnion are everted. In the Lepidoptera no dorsal tube is 

 formed. The embryonic integuments are here simply constricted off from the embryo 

 and serve as the first food of the young grub. This is perhaps also the case in the 

 Diptera and Hymenoptera. 



To return now to the developmental processes in the embryonic rudiment itself, 

 we must go back to Fig. 350, B. The blastopore here appears closed. The invagin- 

 ated tube (germ streak) is compressed dorso-ventrally and has a slit-like lumen. The 

 transverse section C of a still later stage shows us the germ-streak spread out flat 

 under the ectoderm. On each side of the middle line the latter is thickened and 

 bilaminar. The thickened parts are transverse sections of longitudinal thickenings 

 (primitive thickenings), between which there is a shallow median longitudinal groove 

 (primitive groove). The deeper cells of the longitudinal thickenings form the two 

 lateral strands, from which come the paired portions of the ventral chord. The so- 

 called yolk furrowing has taken place at this stage, the yolk belonging to each 

 nucleus being marked off, so that the whole is divided into irregular masses. 



In the transverse section D we see the germ streak divided into two lateral halves, 

 a cavity appearing in each half ; these cavities become those of the primitive 

 segments. These appear more clearly demarcated in the transverse section E of 

 an older stage. They are the mesoderm cavities, which are repeated segmentally on 

 each side. The remaining mesoderm of the germ streak has again united in the 

 middle line. On each side near the primitive thickenings the ectoderm becomes in- 

 vaginated to form a trachea. These tracheal rudiments appear segmentally in pairs, 

 as is shown in the ventral surface view, Fig. 353, where their outer apertures 

 (stigmata) are seen. 



In the transverse section F of a later stage the germ streak has drawn back some- 

 what from the surface of the yolk, and so gives rise to a cavity, which becomes the 

 definitive coelome and later joins the segmental cavities. The longitudinal trunks of 

 the tracheae are already formed, as shown in transverse section. The lateral strands 

 of the nervous system have separated from the ectoderm (hypodermis), and the 

 primitive groove between them has deepened. The fusing of its base with the lateral 

 strands yields the transverse commissures of the ventral chord. We find in each 

 side between the yolk and the segmental cavities a newly formed layer of cells. This 

 cell layer represents the endoderm. Gradually extending all over the surface of the 

 yolk, it becomes the epithelium of the mid-gut which encloses the yolk. The yolk 

 is gradually absorbed later. The wall of the mesoderm cavities which is in contact 

 with the endoderm follows the latter, as it grows over the yolk and yields, as the 

 visceral layer of the mesoderm, the muscular wall of the mid-gut. 



Long before all these processes have taken place the stomodseum has formed at 

 the anterior (head) end of the embryonic rudiment and at the posterior end (in the 

 terminal segment) the proctodaeum, both being ectodermal invaginations which 



