GASTRULATION IN VARIOUS CHORDATA 



429 



(k) Somitic mesoderm (i.e., the mesoderm of the future somites) appar- 

 ently passes inward between the epiblast and hypoblast from the antero- 

 lateral portions of the primitive streak. It migrates forward and be- 

 comes extended along either side of the notochordal cells during the 

 period of primitive-streak recession. The nephric and lateral plate 

 mesoderm involutes along the middle portions of the streak, and this 

 mesoderm becomes extended antero-posteriorly. The hypomeric or 

 lateral plate mesoderm also diverges laterally. The extra-embryonic 

 mesoderm moves inward along the postero-lateral portions of the 

 streak; it migrates laterally and anteriorly (fig. 2021, extra-embryonic 

 mesoderm). 



-LIVE R 



- HEART 



- CHORDA 



- THYROID 

 - NEPHROS 



- INTESTINE 

 -ERYTHROCYTES 

 - MELANOPHORES 

 -SKELETAL MUSCLE 



-SOMITE 

 — ENTODERM 

 -SINUS RHOMBOIDAL 

 ■«■ NEURAL CREST 

 LP-LATERAL PLATE 

 M-HEAD MESODERM 



Fig. 205. Three-germ-layered blastoderm or late gastrula of chick, showing the mosaic 

 distribution of developmental tendencies. (AC after Rawles, '36; D and E after Rudnick, 

 '44, from various sources.) (A-C) The lines transversely placed across embryo are at 

 levels of 0.3 mm. and 0.7 mm. from the center one, considered as 0.0 mm. (A) Ecto- 

 dermal or external layer: neural plate area is indicated in black, epidermal area in white. 

 (B) Mesodermal or middle germ layer. (C) Entodermal or inner germ layer. (D) 

 Ectodermal layer shown on left, mesodermal and entodermal on right. (E) Superficial 

 or ectodermal layer shown at left, deeper layer, at right. (Note: These diagrams should 

 be considered only in a suggestive way; final knowledge relative to exact limits of poten- 

 cies, especially in the mesodermal layer, should be more thoroughly explored.) 



