GASTRULATION IN VARIOUS CHORDATA 



435 



CEPHALIC MARGIN 



^MBRYONIC DISC 



AUDAL MARGIN 



■'■■^ PRIMITIVE GUT '*'*'..'<*.>_, ~^ 



T R PHECTO DE RM 



, _, % 



^ «,^«?«Si3^ *^ EN TODER M PROLIFERATING 



><*' -* *^^^ ME SODERM 



ROPH ECTODERM 



MIGRATING 



MESODERM 



CELLS 



RO PHENTODE R M^jL^. iS 



— — ^^rr"T^»:;rr:^Svv=^^^~;:rr-7-->jr--— — —^-^ mitive streak 



/ y£^°if^'''^f^"^^\ notochord . '"'^~»*»-^^lfj*^''^^4'r>"2:«i-i, 



Vv*^*^--'"*^ t?^ \ NOTOCHORD HENSEN S NODE ^~^— *•'■>.:. ^i'^^A^'-V 



I .' J^llff ^ AMNIOTIC •-'^^v^^, n 



-^' /* %fi pre-chordal mesoblast ■^•■'^■<c« 



/4' plate vitelline '-' 



f ".'• MES08LA ST 



Fig. 209. Longitudinal and transverse sections of the early embryonic (germ) disc of 

 the pig. (C and D after Streeter, '27.) (A) Early, pre-streak, germ disc, showing caudal 

 thickening of epiblast layer. (B) Early streak germ disc, showing thickened caudal edge 

 of disc and beginning migration of mesodermal cells (see fig. 208A). (C) Transverse 

 section through late gastrula, showing three germ layers. Observe that entoderm is 

 attached to either side of median notochordal rod. (D) Longitudinal section through 

 pre-somite, pig blastoderm, showing the relation of notochord to Hensen's node, entoderm, 

 and pre-chordal plate. 



pig embryo, emboly and epiboly are comparable and quite similar to these 

 activities in the chick. 



c. Gastrulation in Other Mammals 



Though the origin of notochordal and pre-chordal plate cells in the pig simu- 

 lates the origin of these cells in the chick, their origin in certain mammals, 

 such as the mole (Heape, 1883) and the human (fig. 207), resembles the 

 condition found in reptiles, particularly in the lizards, where an enlarged 

 notochordal pouch or canal is elaborated by an invaginative process. Conse- 

 quently, in reptiles, birds, and mammals, two main types of presumptive pre- 

 chordal plate-notochordal relationships occur as follows: 



( 1 ) In one group an enlarged notochordal canal or pouch is formed which 

 pushes anteriad in the midline between the hypoblast and epiblast; 

 and 



(2) in others an abortive notochordal canal or primitive pit is developed, 

 and the notochordal cells are invaginated and proliferated from the 



