60 THE FROG 



Exercise 71. — The Forty-eight-hour Chick Embryo. 



(g) Examine a freshly opened embryo in the forty-five- to forty- 

 eight-hour stage, comparing it with the last. Note the blood vessels, 

 the pulsations of the heart, which should be counted per minute, and 

 the extent to which the blastoderm has extended over the egg. Treat 

 with India ink and alcohol as before. Study the specimen thus freshly 

 prepared and a stained and mounted specimen of the same stage. 

 Find all the structures previously observed in the thirty-six-hour 

 stage, and, in addition, observe the cranial flexure, the torsion of the 

 cephalic end of the embryo, the lens of the eye, the auditory vesicles, 

 the tubular heart now bent into an S-shape, the omphalo-mesenteric 

 arteries and the sinus terminalis, the gill arches and slits, and the 

 extent to which the anterior and posterior amniotic folds have devel- 

 oped. Draw this stage, in a figure similar to the last. 



(h) Understand from reading and from demonstrations of the later 

 stages of the chick how the embryo is related in its several stages to 

 the yolk mass and the origin and significance of the yolk sac, amnion, 

 chorion, and allantois, which are called embryonic membranes 

 (Fig. 33). 



Exercise 72. — The 6-mm. Pig Embryo. 



(i) Pig embryos about 6 mm. long from crown of head to rump 

 and about eighteen days old correspond in stage of development to a 

 chick embryo after four days of incubation. They are somewhat more 

 advanced developmentally than a human embryo of comparable length 

 and about five weeks old. Study pig embryos 6-7 mm. long which 

 have been obtained from a slaughter house, preserved, and dissected 

 from their embryonic membranes. Study in a watch glass containing 

 dilute alcohol, using a handlens or the low-power objective with re- 

 flected light. Notice how the body is bent; the head appears triangular 

 in shape because of the two flexures in that region. Identify the eye, 

 olfactory pit, and rudiments of the jaws which form the anterior and 

 posterior borders of the most anterior of four grooves along the side 

 of the head. This anterior slit marks the position of the mouth 

 cavity; the others are the grooves that would be perforated as gill 

 slits in an aquatic vertebrate. The dorsal part of the groove just 

 posterior to the mouth marks the position of the external ear of later 

 development. Parts of the brain can be identified through the trans- 

 parent covering of the head, especially the prominent mesencephalon 

 where the most anterior flexure is noted and the thin-roofed myelen- 

 cephalon which is dorsal to the grooves previously located. The head 

 is folded against the heart, which is the most anterior of three swollen 



