618 PRACTICAL DIRECTIONS CHAP. 



2. Second day. Note a, the parts mentioned under first 

 day, tracing the series of sections through the whole embryo ; 

 b, the gradual meeting of the medullary folds to form the 

 hollow medullary cord ; c, that anteriorly the head has 

 become folded off from the yolk, so that the enteron in this 

 region forms a closed tube, lined all round by endcderm ; 



d, the cerebral vesicles, and the optic vesicles arising from the 

 fore-brain, the lens and the auditory pits from the ectoderm ; 



e, the mesoderm, consisting on either side of dorsal mesodermal 

 segments and of a ventral portion, split into parietal and 

 visceral layers, with the ccelome between them the former 

 layer, with the ectoderm, constituting the body-wall or 

 somatopleure, and the latter, with the endoderm, the wall 

 of the enteron, or splanchnopleure ; f, the heart, vitelline 

 vessels, paired aorta;, and cardinalveins (Figs. 155 and 157, A). 



3. Third day. Note a, the parts mentioned above under 

 the first day, and their further development, including the 

 flexure of the brain, formation of the optic cup or secondary 

 optic vesicle, choroid fissure, and the closure of the auditory 

 pits ; b, the mesonephric (Wolffiari) ducts on the dorsal side 

 of the coslome ; c, in the pharyngeal region, the relations of 

 the visceral arches and clefts and of the arterial arches ; d, the 

 somatopleuric amniotic folds, and the way in which they 

 meet to form the serous membrane ('false amnion") and 

 the true amnion and amniotic cavity. (The allantois arises 

 from the hinder part of the splanchnopleure on the fourth or 

 fifth day). 



(Figs. 157, B, C, 160, 161, and 164.) 



D. i. If you have been fortunate enough to obtain some 

 blastocysts of the rabbit (see p. 549), examine them carefully 

 with a lens, and note the outer layer of cells (trophoblast) and 

 the smaller mass (embryonic area) attached to the former on 

 the inner side of the vesicle at one pole (Fig. 154, E). 



2. Obtain, either fresh or preserved, a gravid uterus of 

 the rabbit or rat containing advanced embryos. Slit open 

 each swelling of the uterus carefully on the ventral side, 

 and note a, the flattened umbilical vesicle (" yolk-sac ") ; 

 b, the fcetus, enclosed by the thin, transparent amnion, 

 containing a fluid ; c, the vascular discoid placenta, closely 

 attached to the dorsal side of the uterus, and connected 

 with the navel or umbilicus of the foetus by a vascular 

 umbilical stalk, consisting of the stalk of the allantois and 

 that of the umbilical vesicle twisted around one another 

 (Figs. 167 and 168). 



3. Sections should be made through the uterine wall and 

 placenta in order to make out the relations between the 

 maternal and foetal portions of the placenta (Fig. 168). 



