38 Growth in length of the Vertebrate Embnjo 



Lieberkiihn. This area which " invaginates " is said to be endo- 

 dermal and since it clearly does not invaginate at the posterior 

 end, for there is no groove there it constitutes the equivalent of 

 a yolk plug. Therefore after invagination we have the relation 

 as shown in Fig. 20. 



Van Beneden tries to show that the whole of the notochord 

 and gut lining is derived from this archenteric rudiment and he 

 brings forward some very remarkable evidence from the corre- 

 sponding stages in the Bat in support of this. But even with 

 sections which favour his views there still remains a considerable 

 amount of gut lining which must be derived from the "lecitho- 

 phore." 



Thus Fig. 21 which is taken from Fig. 15 of his Plate XXI. 



Fig. 21 after van Beneden. Bat. 



Sagittal section through the anterior end of an embryo with six protovertebrae. 

 ar pi remains of archenteric plate projecting into the foregut ; lee lecithophore. 



It is however not at all certain that the remnant of archenteric 

 plate has not shifted its position. It is degenerating tissue and 

 may quite well move, as I have seen degenerating trophoblast 

 move in the Pig. 



Again the pericardium is very clearly indicated at quite an 

 early stage by the thickening of the inner layer (lecithophore of 

 van Beneden) and this layer certainly occupies the position 

 marked lee in the above figure. 



