30 



EARLY CHANGES IN THE BLASTODERM. 



EARLY CHANGES IN THE BLASTODERM, RESULTING IN 

 THE FORMATION OF THE EMBRYO. 



FIRST APPEARANCE OF THE EMBRYO; FORMATION OF THE NEURAL GROOVE 

 AND MESOBLASTIC SOMITES. 



Neural canal. The blastoderm in mammals, as we have seen, eventually 

 completely encloses the cavity of the ovum, and even in the large telolecithal ovum 

 of the bird, it gradually extends so as to cover a large part of the yolk. But only 

 a small portion of this membrane takes part in the formation of the body of the 

 embryo, that portion namely which lies immediately in front of the primitive groove, 

 and it is here that what may be regarded as the first trace of the embryo makes its 

 appearance very soon after the outgrowth of mesoblast from the sides of that 



Fig. 30. EMBRYONIC AREA, WITH OUTLINE OF PART or THE VASCULAR AREA, FROM A RABBIT'S 

 OVUM OF SEVEN DAYS. ~*. (From Kolliker.) 



oo, vascular area ; ag, embryonic area ; pr, primitive streak and groove ; rf, medullary groove. 



groove in the form of a shallow furrow (fig. 30, rf), wide behind where it 

 embraces the anterior end of the primitive streak, and at first narrow in front, and 

 bounded on either side and anteriorly by a fold of epiblast ; which folds, in fact, 



Fig. 31. SECTION ACROSS THE ANTERIOR PART OF THE MEDULLARY GROOVE OF AN EARLY EMBRYO 



OF THE GUINEA PIG. (E. A. S.) 



ep, folds of epiblast rising up on either side of the middle line, and thus bounding the medullary 

 groove ; mg, middle of medullary groove ; hy, hypoblast, which is in contact with the medullary 

 epiblast at the middle of the groove, but is elsewhere separated from it by mesoblast, me, which has 

 burrowed forwards between the two primary layers. A cleft is seen in the mesoblast on either side ; 

 this is the commencement cf the anterior part of the ccelom. 



