ANIMAL POLE 

 O 



ANIMAL POLE 



VEGETAL POLE 



VEGETAL POLE 



ANIMAL POLE 



ANIMAL POLE 



o 



VEGETAL POLE 



VEGETAL POLE 



Fig. 215. Direction of entodermal projection in relation to egg polarity during gas- 

 trulation in various Chordata. (A) Amphioxus. (B) Frog. (C) Urodele amphibia. 

 (D) Chick. For diagrammatic purposes, the positions to the right of the median egg 

 axis in the diagrams arbitrarily are considered as clockwise positions, whereas those to 

 the left are regarded as counterclockwise. 



Pig. 216. Exogastrulation in the axoiotl (Amphibia). (From Huxley and De Beer, '34, 

 after Holtfreter: Biol. Zentralbl., 53: 1933.) (A, B) Mass outward or exogastrular 

 movements of entoderm and mesoderm, resulting in the separation of these organ- 

 forming areas from the epidermal, neural areas shown as a sac-like structure in upper 

 part of figure. (C) Section of (B). Exogastrulation of this character results when the 

 embolic movements of gastrulation are directed outward instead of inward. Observe that 

 neural plate does not form in the ectodermal area. 



448 



