32 



FORMATION OF MESODERM 



(D) In the kalong (an East Indian bat, Pteropus edulis) (fig. 42, /) the condition is much the 

 same as in the guinea-pig, but the blastocyst remains rounded ; there is no invagination of the 

 trophoblast ; no false amnionic cavity ; and there is not much greater inversion of the layers 

 than occurs in the bats. 



In apes and man (fig. 40) some such condition as in Pteropus in all probability exists ; but 

 there is this difference, that, owing to the great expansion of the trophoblast-shell and to the 

 tardy formation of the entoderm, there is from the first a space between the trophoblastic wall 

 of the blastocyst and the entodermic sac. 



Formation of the trilaxninar blastoderm : mesoderm and em- 

 bryonic axis. The mode of the formation of the middle layer in the Primates 

 varies in certain important particulars from that generally regarded as typical. 

 It will be convenient to give first a brief general description of the mode of 

 formation of the mesoderm in one of the lower mammals e.g. the mole or rabbit. 



The germinal area, at the stage now reached, is a circular disc on the upper 

 pole of the blastocyst (fig. 43). By unequal growth the disc becomes oval, and at 

 its smaller end a linear shading appears which is produced by a keel-like thickening 





FIG. 46, A AND B. VIEWS OP THE EMBKYONIC AREA OF THE BABBIT, SHOWING TWO STAGES IN THE 



EXTENSION OF THE MESODERM. (Kolliker.) 



In A the mesoderm extends on either side of the primitive streak over the posterior part of 

 embryonic area, and also behind the primitive streak beyond the limits of that area. 



In B the mesoderm extends over a circular area which surrounds the embryonic area. The 

 embryonic area is also trilaminar, except in the middle line in front of the primitive streak. 



of the ectoderm, known as the primitive streak (figs. 44 and 45). The first part 

 of the streak to appear is a knob-like thickening which forms its head (Hensen's 

 knot). From the primitive streak cells are budded off into the space between 

 ectoderm and entoderm. They form a loosely arranged layer of branched elements 

 named the mesoderm. The ectodermic thickening, at first separate from the 

 entoderm, quickly fuses with it, so that all three layers are continuous in the 

 primitive streak. By constant proliferation the mesoderm spreads round the 

 wall of the blastocyst until finally it entirely surrounds it. It will be noticed 

 that at first the extension is mainly backwards in a continuous sheet behind 

 the germinal area (fig. 46). This portion of the layer takes no part in the forma- 

 tion of the embryo, but is concerned in the laying down of the peripheral 

 mesoderm of the future vascular area. Within the germinal area the sheet is 

 divided into two lateral wings, separated by the primitive streak from which they 

 spring (fig. 58, p. 40 ; fig. 66, p. 44 ; and fig. 47). As the mesoderm continues to 

 spread, the embryo begins to take form in front of the primitive streak, and the 

 lateral wings of the mesoderm are found extending forwards on each side of a 



