K()\VL TIIIK]) DAY 



543 



number of blood-vessels such as ventral and dorsal carotids and 

 anterior cardinal veins. 



As will be gathered by sliding a straight-edge forward over the 

 figure of the whole embryo, its edge parallel to the plane of the 

 sections, there conies a point in the series where the sections through 

 the reflexed part of the head and the rest become continuous. This 

 happens as soon as the deep niche in the bend of the head is passed. 

 Such a section is represented in Fig. 237c. Comparison of this 

 figure with the preceding one will make clear the fact that the 

 extreme ends of the section are both of them morphologically dorsal. 

 The brain is cut through twice on the right of the figure is the hind 

 brain while on the left is the thalamencephalon distinguished by 



spl: 



FIG. 238. Diagrammatic sagittal section through third-day Fowl embryo. The notochord 

 and dorsal aorta are omitted Ectoderm and endoderm are indicated by continuous 

 lines, mesoderm (except endocardium) by dots. 



a, position of anus, not yet perforate ; all, allantois ; am, aninion ; nt, atrium; a.e, amniotic edge; 

 /.a, serous membrane ; f.g, foregut; I, lung rudiment; HUSS, mesencephalon ; pa.g, postanal gut; j-t, 

 pituitary involution ; //*, rhombeneephalon ; .s-y/7, splanchnopleure of yolk-sac; t, tlialamcm-ephalon ; 



th, thyroid; /', ventricle; v.A, ventral aorta; r.-m. remains of velar membrane; i/.s, cavity of yolk- 

 sac : //..-/. cavity of yolk-stalk. 



the pocket-like rudiment of the pineal organ (pin). The thin optic 

 stalk lies outside the section, but the structure of the optic cup 

 otherwise is well seen. The lens is in the form of a closed vesicle 

 which has by this stage become completely nipped off from the 

 external ectoderm. Immediately ventral to the thalamencephalon is 

 the pituitary involution cut transversely- The section passes through 

 the ganglia of the auditory nerve (gang) and on the embryo's right 

 through the nerve root connecting the ganglion with the medulla 

 oblongata. Various blood-vessels are cut through : their names and 

 relations with one another are most easily determined by sliding a 

 straight-edge along the drawing of the embryo as a whole. 



The study of this stage should be completed by examining series 

 of sections parallel to the sagittal plane in the head region and 



