362 THE RABBIT. 



thin lateral sheets of cells (Fig. 142. M), which lie between the 

 epiblast and hypoblast, and which give rise to the middle 

 germinal layer or mesoblast. In the primitive streak itself the 

 cells are spherical and closely compacted ; but in the lateral 

 mesoblast sheets the cells are more loosely arranged, and are 

 stellate in shape. 



The layer of mesoblast spreads rapidly, both laterally and 

 posteriorly ; at the end of the seventh day, its limits are indicated 

 by the shaded area bounded by the dotted line, M, in Fig. 143, 

 a figure that may with advantage be compared with Fig. 107, 

 which shows the corresponding stage in the development of the 

 chick. 



While it is certain that the mesoblast in the posterior part 

 of the embryonal area of the rabbit, i.e. in the region of the 

 primitive streak, arises in the manner just described, by pro- 

 liferation of cells of epiblastic origin, it is by no means clear 

 that the whole of the mesoblast is formed in this- way ; and, 

 although further observations are wanted on the point, it seems 

 probable that in front of the primitive streak, in the part 

 of the embryonal area in which the embryo will appear, the 

 mesoblast arises, as in the chick, by budding off of cells from the 

 hypoblast. 



GENERAL HISTORY OF THE DEVELOPMENT OF 

 THE RABBIT EMBRYO. 



In the preceding section the development of the rabbit's 

 ovum has been followed up to the end of the seventh day, that 

 is, up to a point corresponding to that reached by a hen's egg 

 about the sixteenth hour of incubation. At this stage all three 

 germinal layers, epiblast, mesoblast, and hypoblast, are estab- 

 lished ; a primitive streak and primitive groove are present ; but 

 there is as yet no trace of the embryo itself. 



It will be convenient to give, in the present section, a brief 

 summary of the mode of formation, and of the general course 

 of development of the embryo, before considering in detail the 

 history of the several systems and organs. 



1 . The Formation of the Embryo. 



The embryonal area of the blastodermic vesicle of the rabbit 

 at the end of the seventh day (Fig. 143. AD) corresponds very 



