30 Growth in length of the Vertebrate Embryo 



tinguishable from each other. In some mammals it may be 

 different, as in Manis and Galeopithecus, where it is said that the 

 trophoblast is to be made out before the ectoderm and endoderm 

 are distinguishable from each other. 



Then the circular ectoderm plate becomes bent inwards and by 

 its growth causes the rupture of the trophoblast above it, thus 

 allowing the plate to appear on the surface; pieces of trophoblast 

 upon the surface degenerate and disappear. 



All this time the symmetry has been markedly radial or 

 spherical. The cavity of the blastocyst becomes the gut, or at 

 any rate the fore gut is derived from it. It is the protogenetic 

 enteron or true archenteron. There is no blastopore. Up to now 

 everything that has happened has been the result of protogenesis 

 and the result is on the whole spherical or radial. 



In the Babbit the history up to this point is much the same, 

 the chief points of difference being: 



(i) The egg contains protein not fat. 



(ii) There is a very tough albuminous layer laid on during 

 the passage of the ovum down the Fallopian tube, and this acts 

 like the leather coat of a football in keeping the more distensible 

 inner vesicle taut, and also it prevents the embryo (blastocyst) 

 from coming in contact with the walls of the uterus for some 

 considerable time. 



(iii) There is no trace of inversion. The rupture of the 

 overlying trophoblast is general. 



(iv) The endoderm does not completely surround the inner 

 surface of the trophoblast. 



(v) The archenteron arises more as a split between cells than 

 as vacuolations within cells. 



The failure of the endoderm to line the ventral part of the 

 blastocyst looks as though there were no ventral wall to the gut 

 cavity if as some embryologists think the trophoblast is 

 ectodermic. Of course if it is endodermic or homologous with 

 yolk cells, then there is no such difficulty. 



We can now consider the next stage in development. Proto- 

 genesis has come to the end of its solitary reign. The radial 

 symmetry to which it gave rise now becomes profoundly modified 

 by the origin of a secondary growth centre resulting in growth 



