GASTKULA THEORY 



45 



over the volk. The entoderm is produced by delamination (first stage of gastrulation). It is from 

 the first entirely within the theoretical bJastula, which is completely filled, as it were, by the ento- 

 meres and the great mass of unsegmented yolk. Owing to the profoundly altered conditions, the 

 second phase of gastrulation is further modified and abbreviated, as will be explained presently. 



In the Mammalia the egg is holoblastic, but the blastocyst does not represent a blastula 

 like that of Amphioxus, but is rather like the theoretical blastula of one of the lower Amniota, 

 empty of yolk and with one portion of its wall invaginated. The entoderm is formed by 

 delamination, but when the embryonic ectoderm-plate is differentiated a circular blastopore 

 such as is seen in none of the lower Amniota in some cases breaks through. 



Now, analysing the early appearances in the primate blastoderm in terms of the gastrula 

 theory and in the light of the above interpretations, the posterior edge of the germinal area 



FIG. 67. MEDIAN LONGITUDINAL SECTION OP A BLASTODERM OP VESPERTILIO MUBINUS, SHOWING THE 

 NOTOCHORDAL CANAL BEFORE ITS OPENING. (Van Beneden.) 



n.c., posterior opening of notocho rd- canal ; n'.c',, its anterior opening ; x>.s., primitive streak ; 



n.p., notochord-plate. 



from which the mesoderm first springs would represent the ventral lip of the blastopore, and 

 Hensen's knot its dorsal lip. Hensen's knot, however, signifies the commencement of active 

 changes in the dorsal lip which are concerned in the formation of the embryonic axis, and before 

 these are initiated a continuous layer of unpaired, and always unsegmented mesoderm is laid 

 down from the lips of the blastopore, which has no part in the embryo except at its extreme 

 head end. The formation of the primitive streak, during which the posterior border of the 

 disc or ventral lip is carried away from Hensen's knot by the growth of the pointed posterior 

 end of the shield, would signify the drawing out of the gastrula-mouth into a lineal aperture of 

 which the lips are fused. The second phase of gastrulation is now entered on, during which a 

 true invagination, but of a modified character, appears to occur. It is associated with the 

 backward growth of the dorsal lip and the laying down of the dorsal plate (embryonic axis) 



n'.c'. 



o.n. 



FIG. 68. MEDIAN LONGITUDINAL SECTION OF A BLASTODERM OF VESPERTILIO MUBINUS, SHOWING THE 



NOTOCHORDAL CANAL AFTER THE BREAKING THROUGH OF ITS FLOOR. (Van Beneden.) 



c.n., neurenteric canal ; c, anterior persisting portion of the notochordal canal ; 

 other letters as in fig. 67. 



in front of it, and the re-establishment of the blastopore in the formation of the notochordal 

 canal. 



In the Reptilia processes are observed which throw a light on the much more modified 

 phases in mammals, and can again be linked on to the processes seen in those amphibian eggs 

 which are heavily yolked. On the posterior border of the germinal area a thickening appears 

 called the primitive plate. On this a slight pocket appears, marking a spot where ectoderm 

 and entoderm are indistinguishable. The appearances are suggestive of proliferation of cells 

 to form the entoderm, but there is no true invagination. Later (second phase of gastrulation), 

 at the anterior end of the primitive plate the pocket deepens to form a true invagination, which, 

 as the dorsal lip increases in length to form the dorsal plate, becomes extended into a canal. 

 The floor of the canal fuses with the primitive or yolk entoderm. and then breaks through, so 



