46 



THE FIEST DAT. 



[chap. 



larity, so placed that its long axis forms a right angle, or 

 very nearly a right angle, with the long axis of the egg 

 itself. Its narrow end corresponds with the future hind end 

 of the embryo : and we may henceforward speak of it as the 

 hind end. If an egg be placed with its broad end to the 

 right hand of the observer, the head of the embryo will in 

 nearly all cases be found pointing away from him. 



At about the time when the pellucid area is beginning 

 to undergo this change of shape, there appears in surface 

 views, along a line corresponding with the long axis of 

 the oval, and occupying not, as might perhaps be expected, 

 its front but its hinder two-thirds, a narrow opaque streak, 

 much more opaque, and therefore distinct, than the em- 

 bryonic shield, but still shadowy and ill-defined. This is 

 known as the primitive streak. 



Fig. 10. 



Section of a Blastoderm at bight angles to the long axis of the 



Embryo after eight hours' Incubation. 



(Taken about midway between front and hind end.) 



A. epiblast. B. mesoblast. C. hypoblast, pr. primitive groove. /. fold in 

 the blastoderm, probably produced by the action of the chromic acid. 

 m. c. mesoblast cell ; the line points to one of the peripheral mesoblast 

 cells lying between epiblast and hypoblast, bd. formative cells. 

 The following are the chief points represented in the section, (i) The 

 thickening of the mesoblast underneath the primitive groove pr., even when 

 it is hardly at all present at the sides of the groove. (2) The hypoblast, C, early 

 formed as a single layer of spindle-shaped cells. (3) The so-called segmentation- 

 cavity, in which coagulated albumen is present. On the floor of this are the 

 large formative cells bd. 



The line of separation between the epiblast and mesoblast underneath the 

 primitive groove is too strongly marked in the figure. 



The primitive streak is no sooner formed than it becomes 

 marked on its upper surface by a delicate shallow furrow 



