CHAP, xiii DEVELOPMENT OF RABBIT 585 



F, vertical section of blastoderm and adjacent part of the egg at the time of 

 laying, before incubation : the anterior edge of the blastoderm is to the right 

 the posterior edge to the left of the figure (x 25) ; G, transverse section 



o ge o e e o e gure x 25 ; , transverse section 

 of blastoderm at about the twentieth hour of incubation, the section passing 

 through the primitive streak and groove at about the middle of its length 

 x 200. 



(x 200). 



mesoerm ; me. gr. meuary groove ; pg. primitive groove ; pr. am. pro- 

 amnion ; pr. st. primitive streak ; y. yolk ; zf. formative cell ; zl. lower layer 

 cells. (A and B, from Foster and Balfour ; C G, from Marshall, after Duval 

 and Coste. 



. 

 and Coste.) 



endoderm only until the end of the second day of incubation, 

 and by the third day the whole blastoderm has become 

 three-layered. 



Before tracing the development of the various organs 

 derived from the three germinal layers respectively, it must 

 be remembered that the actual chick embryo is formed from 

 the area pellucida, and gradually becomes folded off from 

 the yolk, while the area opaca extends over the yolk until 

 the latter is entirely covered by it (iyth day of incubation) : 

 thus we can distinguish between an embryonic and an 

 extra-embryonic portion of the blastoderm. In the meso- 

 derm of the latter portion blood-vessels are formed, so that 

 it is then known as the area vasculosa (see Fig. 159). 



The minute egg of the rabbit (compare p. 574) and 

 of most other mammals, although microlecithal and 

 undergoing a holoblastic segmentation, has presumably 

 been derived from a meroblastic type with abundant 

 yolk, like that of the bird, and some lower mammals 

 (Ornithorhynchus and Echidna) living in Australia at the 

 present day still possess eggs of this type. In the higher 

 Mammalia the yolk has disappeared, as it is no longer 

 needed, the embryo, as we have seen (p. 483), being 

 nourished by means of a placenta, which will be described 

 presently. The early processes of development are 

 therefore somewhat peculiar, and though the segmenta- 

 tion is holoblastic, the subsequent development is for 

 some time essentially similar to that of the bird : it is 

 not until later stages that the more special characteristics 

 of the mammal on the one hand and the bird on the 

 other become apparent. 



PRACT. ZOOL. O O 



