xii PRACTICAL DIRECTIONS 605 



scissors, float it off in the water, preserve, stain, and mount entire in 

 Canada balsam. (The removal and preparation of the blastoderm at this 

 stage is rather difficult, and sections of the next stage will illustrate the 

 chief points equally well). 



3. End of first day (aboztt 24 hours]. Examine as before, com- 

 paring Fig. 155, B. Sketch, and then preserve, stain, and eventually 

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

 and its gradual closure to form the medullary cord ; the neural ridges 

 (p. 586) ; the notochord', the parietal (somatic] and visceral (splanchnic} 

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

 Sketch one or two typical sections. 



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

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

 pits, heart, and vitelline vessels, paired aorta and cardinal veins, meso- 

 nephric ducts, and amnion, as well as the increase in the number of 

 mesodermal segments (compare Figs. 157, 158, 159, 160, 161, and 

 164). 



5. Third day. Examine, prepare, sketch, and if time permits, cut 

 sections, noting, in addition to the points already referred to, the further 

 gradual folding off of the embryo from the yolk (compare p. 471) and' 

 extension of the amnion and area vasculosa ; the cerebral flexure ; the 

 visceral arches and clefts ; the arterial arches ; the mesonephric tubules ; 

 and the further development of the eye and ear (compare Figs, referred 

 to in 4). 



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

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

 Figs. 164-166). Sketch. 



7. Examine and compare a few later stages. 



E. Obtain some advanced embryos of the rabbit or rat, which 

 should be examined in situ in the uterus before they are removed and 

 preserved (p. 549)' Note the amnion, and examine the placenta and 

 its connection with the uterine wall and with the foetus. Sketch. 



