XI PRACTICAL DIRECTIONS 587 



3. End of first day (about Zi, hours). Examine as before, com- 

 paring Fig. 145 B. Sketch, and then preserve, stain, and eventually 

 cut sections. Note the eciodertn, endoderm, mesoderm, medullary groove 

 and its closure to form the medullary cord ; the neural ridges (p. 568) ; 

 the notochord ; the iarietal [somatic] and visceral [splanchnic] layers 

 of the mesoderm, and the ccelome (compare Figs. 64, 65, and 147). 

 Sketch one or two typical sections. 



4. Second day. Examine, prepare, cut sections, and sketch as before, 

 noting the fore-, mid-, and hind-brain, the optic vesicles, lens, auditory 

 pits, heart and vitelline vessels, and the head-fold of the amnion, as well 

 as the increase in the number of mesodermal segments (compare P'igs. 

 148, 149, and 153). 



5. Third day. Examine, prepare, and sketch as before, noting, in 

 addition to the points already referred to, the further gradual folding 

 off of the embryo from the yolk (compare p. 454) and extension of the 

 amnion and area vasculosa ; the cerebral flexure ; the visceral arches 

 and clefts and the arterial arches ; the mesonephric dud ; and the 

 further development of the eye and ear (compare Figs. 147, 148, 149, 



■ 150, I S3, and 155). 



6. Fourth to sixth days. Observe the further development of the 

 parts already seen and also the limb-buds and the allantois (compare 

 Figs. IS3-ISS)- Sketch. 



I. Examine and compare a few later stages. 



E. Obtain some advanced embryos of the rabbit (p. 542) or rat. 

 and examine in situ in the uterus before removing and preserving them. 

 Note the amnion and examine the placenta and its connection with the 

 uterine walland with the foetus. Sketch. 



