DEVELOPMENT OF MAMMALIA. 425 



blood-corpuscles are non-nucleated; one aorta alone exists — 

 namely, the left — which turns over the left bronchus, not the 

 right as in the bird. Lastly, the Mammalia never have those 

 curious respiratory prolongations, the air-sacs, so characteristic 

 of the Class Aves. Although there are marked dilferences 

 between the sauropsids and mammals, yet we have intermediate 

 and connecting links in the peculiar Monotremes of Australasia. 

 We need not refer further to the anatomical features of the 

 Mammalia : the reader is referred to the chapter on the Anatomy 

 of the Horse for fuller details, whilst the peculiarities of each 

 group will be pointed out as we proceed. 



Development of a Mammal and the Fcetal Membranes. 



Mammals, unlike birds, possess two ovaries, as described in 

 chapter xii. In the early stages there is much resemblance 

 between the ova of a bird and a mammal. There are many 

 very important differences, however, later. The Bird's egg is 

 large and full of yolk; the Mammal's is small, and has a scanty 

 supply of yolk. The embryo bird can live and grow on the 

 food stored in the egg, but the fcetal mammal has to depend on 

 maternal food derived from the parent's blood by means of the 

 structure called the placenta, and later from the milk. There 

 are two periods of mammalian development : (i) uterine de- 

 velopment, (ii) post-uterine development. The first is the 

 more important. Mammals produce viviparously, birds ovip- 

 arously. The primitive ova are budded off from the epithe- 

 lium covering the ovary. In the solid part of the gland — the 

 stroma — are vesicles of all sizes, the Graafian follicles. Each 

 follicle has a wall of its own, and contains one ovum (rarely 

 two). The epithelium of the foUicle forms two layers, an 

 external one, the membrana granulosa, and another mass pro- 

 jecting into the vesicle and surrounding the ovum — the 

 discus proligenis; the cavity of the Graafian follicle is full of 

 liquid. As these follicles become ripe they project from the 



