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mesoderm 



Text-Figure 1. 

 nmtic. ) 



Ilhistratiug the formation of the mesoderm. (Diagram- 



EXPLANATION. 



A. Early blastodisc stage, hi. p., the blastopore-like depression in the 

 middle of the eetoblast, eet. msd., mesoderm cells, proliferating from the 

 point just beneath the blastopore-like depression. The accumulation of 

 these cells, and their accumulation beneath the eetoblast, forms the primitive 

 cumulus, y. c, yolk cells ; certain of the proliferating cells which find their 

 way into the yolk, ylk., yolk. 



B. Longitudinal section through the caudal (secondary) thickening. 



1)1. p., blastopore-like depression, c. th., the caudal thickening. The point 

 of most rapid proliferation is in the region of the blastopore, cct., ecto- 

 derm. The eetoblast of the blastodisc has become a distinctive germ layer. 

 msd., cells of the growing mesodermal layer spreading out under the ecto- 

 derm y. c, yolk cells. 



C. Distribution of the mesodermal cieiuents viewed from the under side, 

 c. ///.. caudal tbickeiiing. 



