Early Stages of the Rabbit 31 



in length. The conditions in the Rabbit are such that the plane 

 of the deuterogenetic area coincides at first with that of the 

 protogenetic area, instead of being at right angles to it as in the 

 Anamnia. 



This is (i) partly on account of the great distension of the 

 archenteric space, and (ii) partly because the deuterogenetic 

 centre first shows itself as a circular spot instead of a ring. 



In the Rabbit we have the stages of development of the two 

 centres shown in quite a diagrammatic form in Fig. 16. 



During the seventh day the circular embryonic area is marked 

 out very distinctly in front, but less distinctly behind. 



o 



Fig. 16. Rabbit seventh and eighth day. Diagram 



to show the secondary growth centre. 



The white areas are protogenetic, the shaded areas are deuterogenetic, the doubly 

 shaded portions represent the primitive streak. 



The embryonal area is coterminous as yet with the ectoderm. 

 The surrounding superficial cells are all trophoblastic. Internally 

 the endoderm lines both the trophoblast and ectoderm. About 

 the time that the deuterogenetic centre begins to appear, there is 

 a slight thickening of the endoderm just under the anterior and 

 lateral margins of the ectoderm. This is the first sign of any 

 mesodermal formation, but no cells are actually budded off here 

 until the deuterogenetic tissues have become apparent. This 

 semicircle is however the first sign of the mesoderm from which 

 the heart and pericardium will be developed, but we will confine 

 ourselves at present to the external characters, that is to say 

 to the general form. The deuterogenetic centre begins as a 



