FORMATION OF THE BLASTODERM. 



third or intermediate layer. This is mainly derived, as was first pointed out by- 

 Kolliker, from the primitive ectoderm of the groove, but since in this situation the 

 two primary layers become eventually more or less blended, it is probable that the 

 primitive entoderm cells also take part in its formation, although this part is in the 

 mammal evidently a subordinate one. It is further maintained by many embry- 



ep. 



g. 21. SECTION ACROSS THE POSTERIOR END OP THE EMBRYONIC AREA OF A RABBIT AT THE TIME 



OP THE FIRST SIGN OP A PRIMITIVE STREAK. (Kolliker. ) 



ep, epiblast ; ax, its axial part undergoing proliferation (this is shown by the karyokinetic figures, 

 ; me, mesoblasb becoming derived from the proliferating axial epiblast ; Ay, hypoblast. 



Fig. 22. LONGITUDINAL SECTION THROUGH THE MIDDLE LINE OP PART OP AN EMBRYONIC AREA (MOLE) IN 



WHICH THE PRIMITIVE STREAK HAS BEGUN TO FORM. (Heape.) 



The blastoderm is perforated in front of the (short) primitive streak (? blastopore, Up) ; a few meso- 

 tyast cells are seen anterior to the perforation ; ep t epiblast ; hy } hypoblast ; p.sk, primitive streak. 



Fig. 23. Two SECTIONS ACROSS THE EMBRYONIC AREA OP A BLASTODERM AT THE STAGE SHOWN IN 



FIG. 19. (Heape.) 



A. Section across the anterior end of the primitive streak and groove. 



B. Section across the posterior enlargement of the primitive streak. The epiblast and hypoblast 

 are seen to be united along the primitive streak, p.sk ; laterally the mesoblast, w., the cells of which 

 have grown out from the uniting column of axial cells, separates the two primary layers. 



p.gr, primitive groove ; ep, epiblast ; hy, hypoblast ; m, mesoblast. 



