160 WALTER HEAPE. 



Figs, 20—33, Stage H.— Transverse sections through three embryos of this 

 stage. 



Fig. 20 is a section through the front of the head ; it passes through the 

 point of origin of the optic vesicles, and shows at the same time the 

 pore through which the neural canal is open to the exterior at this 

 stage. 



Fig. 21 passes through both the mid- and fore-brains and through the 

 centre of the optic vesicles, which are here seen to be directed out- 

 wards, downwards, and backwards. 



Fig. 22 passes through the hind-brain and the front end of the fore-gut 

 {al. c). The notochord is not yet separated from the axial hypoblast 

 here. The front edge of the first aortic arch is shown. This vessel is 

 very wide, and may be seen for many sections. 



Fig. 23 is also a section through the hind-brain, but at its posterior end. 

 The notochord is here isolated from the hypoblast. The two grooves 

 in the ventral epiblast on either side the middle line, which were seen 

 in Fig. 22, have met in this figure and form a single deep groove 

 closely in contact with the ventral wall of the fore-gut, and here the 

 mouth will be formed. These grooves define the anterior border of the 

 first visceral arch {vs. acL). The alimentary canal in this figure is 

 very considerably wider than in Fig. 22. In Figs. 20 to 23, the head 

 of the embryo is folded off from the yolk-sac. 



Fig. 24 is taken from a different embryo from what Figs. 20 — 23, and 25 

 are taken. The front end of the heart is shown. The section is not 

 quite transverse, and the first aortic arch is shown on the right side 

 and not on the left side. The extremely wide fore-gut and the separa- 

 tion of the heart into two portions, shown here, is also due to this fact. 



Fig. 25. The alimentary canal is here open ventrally. In Fig. 24 the 

 splauchnopleure had formed a complete layer, but the somatopleure had 

 not met below the gut. In this figure the splauchnopleure as well as 

 the somatopleure are still divergent. The heart is here in the form of 

 two tubes, and the two layers of which it is formed may here be seen. 

 The formation of blood -corpuscles from stellate mesoblast cells also 

 may be observed. The thickened epiblast on either side the neural 

 canal is the commencement of the auditory organ. 



Fig. 26. A section immediately in front of the first protovertebra. The 

 vitelline vein is seen in the splauchnopleure, branching out over the 

 yolk-sac. The fold of the somatopleure to form the amnion is also 

 indicated {am.). 



Fig. 27. A section through the anterior protovertebra. 



Fig. 28. A section through the middle of the embryo. The large vitel- 

 line vessels are shown in the splanchnic layer of mesoblast over the 

 yolk-sac. The true {am.) and false {am.fls.) amnion are both shown 



