198 EDWARD MCCRADY, JR. 



The erythrocytes at birth and for some days afterward are 

 oval and nucleate like those of lower vertebrates. 



XIV. APPENDIX 



Postnatal development. Some experimental techniques. The 

 evolution of the mammals. 



Postnatal development. As its title indicates, the proper province 

 of this book does not extend beyond the time when the newborn young 

 reaches the pouch and becomes attached to a mammary nipple. But 

 though embryology proper may be said to end at that time, so much 

 of development remains to be accomplished that a few words about 

 the pouch young seem necessary before closing. 



The earliest pouch young (i.e., the newborn opossum described in 

 the preceding chapter) averages about 11 mm. in crown-rump length, 

 though many individuals may be as small as 10 mm. C.B.L. It has 

 a fairly well-developed circulatory system; lungs which are func- 

 tional, though unfinished ; a digestive system which is also functional, 

 but unfinished; a muscular system which is partly functional, but 

 unfinished ; a skeletal system which is almost entirely in the cartilagi- 

 nous and membranous stages; a central nervous system which has 

 virtually no cerebellum (Larsell, '35), but is well developed in the 

 hind-brain and cervical cord; a functional mesonephros and a non- 

 functional metanephros; only the earliest stage of histological sex 

 differentiation in the gonads; non-functional eye and ear, and only 

 non-functional endocrine gland anlagen. 



At this stage of development the young opossum becomes so securely 

 attached to its mother's mammary nipple that it cannot detach itself 

 for some 50 days. And during this interval it brings the development 

 of all its organs to a condition roughly approximating that of most 

 newborn placental mammals. A few details of the changes during 

 this 50-day period will be mentioned. 



About 1 week after birth the metanephros begins to function and 

 the mesonephros shows retrogressive changes. At about the same 

 time the mullerian duct anlage appears. It arises as an invagination 

 from the coelomic epithelium into the ventral border of the extreme 

 anterior end of the mesonephros. A very early stage of the di- 

 verticulum is found in a 17-mm. male specimen lent to me by Dr. C. 

 F. W. McClure of Princeton University. The development of this 

 duct and the part it plays in the formation of the female genital 

 tract have been studied by Dr. J. S. Baxter ('35). The following 

 details are taken from his paper. 



