82 EDWARD MCCRADY, JR. 



The inner wall of the somites is breaking up and pro- 

 liferating the sclerotome. The dermo-myotome remains as a 

 square plate with its edges rounded inward. 



The otic placodes have deepened into well-formed cups. 

 The optic diverticula are becoming stalk-like. 



The forelimb blastema has thickened into a very opaque 

 plate. 



Comparative notes. Rabbit embryos of 8-J to 9 days of age 

 are at a comparable stage of development. In comparison 

 with Minot's specimens 571, 573 and 621, the opossum at stage 

 25 is somewhat more advanced, in differentiation of the hind- 

 brain and cervical cord, the heart, the ear and the somites. 

 And unless their anlagen have been overlooked in the rabbit, 

 the opossum is also ahead with respect to the development of 

 the lung pouches. The rabbit is ahead in having a distinct 

 caudal amniotic fold. 



The lizard of 1.5 mm. length (Peter, '04, N.T. 10) is at the 

 same stage of development of the general body form, the 

 central nervous system, the digestive tract, and the notochord. 

 But the opossum is far ahead in the vascular and urinogenital 

 systems, and in the eye and ear. The lizard is more advanced 

 in the formation of a head amnion, but is like the opossum in 

 still having no caudal amnion. 



A 25-hour chick (7 somites) is like the opossum in central 

 nervous system, digestive tract, vascular system, and in hav- 

 ing no amnion of any sort. The opossum is much ahead of 

 it in development of the urinogenital system. 



Comment on the significance attributed to these compari- 

 sons is deferred to page 89. 



The last third of the ninth day 



Stage 26. Cephalic flexure. Beginning of amnion forma- 

 tion. Fusion of hearts in the ventricular region. Origin 

 of the postcardinal vein. Miscellaneous details. Comparative 

 notes. General comments on comparative data. 



