240 



FERTILIZATION 



appears to be common to the eggs of many species (sea urchin, cyclostomatous 

 and teleost fishes, frog, and mammal) is the contraction of the egg's surface, 

 together with the exudation of various substances from the egg. (See, in this 

 connection, the fertihzation theory of Batailion at the end of this chapter.) 

 It is this behavior of the egg's surface which makes the fertihzation membranes 

 and other egg membranes more apparent; it represents one of the essential 

 and immediate activities associated with egg activation. Separation of the 

 various egg membranes at the time of fertilization appears to be secondary 

 to this primary activity. 



Aside from the immediate functions at the time of fertilization, the activities 

 of the various types of vitelline membranes are concerned mainly with nutri- 

 tional, environmental, and protective conditions of the early embryo. The 

 presence of a fluid in the perivitelline space between the membrane and the 

 developing egg affords a favorable environment for early developmental proc- 

 esses. Moreover, it permits the egg to rotate when its position is disturbed, 

 a proper developmental orientation being maintained. A further accommo- 

 dation is evident in that it permits the developing egg to exude substances, 

 including yolk, into the surrounding area, which may be retained in the im- 

 mediate environment of the egg and later utilized in a nutritional way. If the 

 surrounding vitelline membrane were not present, this material, solid or fluid, 

 would be dissipated. For example, in the early cleavage stage of the opossum 

 or guinea-pig egg, yolk material is discharged into the area surrounding the 

 early blastomeres (figs. 118, 125). The exuded yolk and dissolved substances 

 later come to lie in the cavity within the blastomeres and, thereby, may be 

 used for nutritional purposes. Also, in some forms, such as the opossum, the 

 early blastomeres utilize the zona pellucida as a framework upon which they 

 arrange themselves along its inner aspect during the development of the early 



CUMINGIA 21° WHITAKER- 



CHAETOP. 21° WHITAKER 



SA8ELLARIA 20° F.-FREMIET- 



NERElS 2 1° WHITAKER- 



ARBACIA PUNC 2 I ° W H I T A K E R. _ 

 PARACENTROTUS 2I°WARBURG: 

 ECHINUS 21° SHEARER' 



UNFERTILIZED EGGS 



^PARACENTROTUS 2I°RUNNSTR0M 

 ^'SABELLARIA 20°FAURE-FREMIET 

 ^'/,ARBACIA PUNC 21° WHITAKER 

 = ''',-PAR A CENT ROT US 21° WARBURG 

 "'',ARBACIA PUST 205° WARBURG 

 =c--AMOEBA PROTEUS 20° EMERSON 

 I"^~ ECHINUS MILIARIS 2 1° SHEARER 

 -^^^C NEREIS 2 1° WHITAKER 

 ""^^"CUM INGIA 21° WHITAKER 

 VfROG skin 20° ADOLPH 



CHAETOPTERUS 21° WHITAKER 



FERTILIZED ERGS 



Fig. 120. Effects of fertilization on oxygen consumption in various marine eggs. (After 

 J. Brachet, '50; data supplied by Whitaker.) 



