CHAPTER VIII 



ADAPTATION TO ENVIRONMENTAL CONDITIONS 

 DURING EARLY STAGES OF DEVELOPMENT 



I. Protective Envelopes of the Zygote. — The Zygote or fer- 

 tilized egg is in the Vertebrata as in other groups provided with 

 protective envelopes. Of such we may recognize three fundament- 

 ally distinct types which are conveniently designated as primary, 

 secondary and tertiary envelopes respectively. A primary envelope 

 is a cuticular covering of the surface of the zygote : it is therefore 

 produced by the living activity of the protoplasm of the macrogamete 

 or zygote itself. A typical example of a primary envelope is the 

 vitelline membrane which is formed on the surface of the Echinoderm 

 egg in response to the act of fertilization. The " vitelline membrane " 

 which covers the surface of the egg of a Bird is commonly looked on 

 as a primary envelope. 



A secondary envelope is one which is formed by the activity of 

 the surrounding cells while the egg is still contained in its ovarian 

 follicle. It may be cuticle-like in its nature or it may be composed 

 of cells. 



Finally tertiary envelopes are formed by the excretory activity 

 of the oviducal lining, being deposited upon the surface of the egg as 

 it travels down the oviduct. Of such a nature are the complicated 

 protective envelopes which surround the egg of a Bird or Reptile, 

 or the simpler jelly-like investment found in the case of most 

 Amphibians. 



Apart from tertiary envelopes the most conspicuous envelope of 

 the Vertebrate egg is usually what is known as the zona radiata or 

 zona pellucid'a — the former name being given to it from the fact that 

 it is pierced by numerous very fine canals which give it a character- 

 istic radiate appearance when seen in section. These fine canals 

 apparently contain protoplasmic bridges connecting the protoplasm 

 of the egg with that of the follicle-cells which surround it while still 

 in the ovary, and doubtless having for their function the passing in 

 of food-material from the follicle-cells into the egg-cell. 



The zona radiata is, as a rule, most, conspicuous during early 

 intra-ovarian stages while the egg is undergoing active growth during 

 the storing up of yolk. Later it thins out and becomes less con- 



455 



