310 GLOSSARY OF EMBRYOLOGICAL TERMS 



Yolk — highly nutritious food (metaplasm) consisting of non-nucleated 

 spheres and globules of fatty material found in all except alecithal eggs. 



Yolk Nuclei — darkly staining chromatin-like substances within cytoplasm 

 of young (immature) eggs around which yolk is accumulated during 

 growth phase of oogenesis. May be derived from nucleoli which escape 

 from nucleus. 



Yolk Plug — a plug formed by large yolk cells which are too large to be 

 incorporated immediately in floor of archenteron of amphibian em- 

 bryo, hence are found protruding slightly from blastopore. Size of plug 

 is often used to determine approximate stage of gastrulation. 



Zone, Marginal — presumptive chorda-mesodermal complex at junction of 

 roof and floor of early gastrula. (Syn., germ ring.) 



