196 



affecting subsequent development it evidently 

 has no special significance for the later 

 events. Even when its elevation is inhibited, 

 normal development can ensue (Tyler, '37; 

 Tyler and Scheer, '37). If the unfertilized 

 eggs are treated with such agents as isotonic 

 urea or trypsin, that presumably dissolve 

 the vitelline membrane, fertilization and 

 cleavage can occur without membrane forma- 

 tion (Moore, '30a,b, '32, '49). Of particular 

 interest in this connection is the report by 

 Hultin ('48a,b) that such treatment renders 



Embryogenesis: Preparatory Phases 



changes in volume of the egg upon fertiliza- 

 tion. There are, however, conspicuous 

 changes in shape in many species. Where 

 such changes occur it is invariably in the 

 direction of greater sphericity. Thus many 

 species of echinoids have ellipsoidal, some- 

 what irregular, eggs that become spherical 

 upon fertilization. Such changes undoubtedly 

 reflect changes in the rigidity of the surface 

 and the viscosity of the egg contents (see 

 Tyler, '32 and Runnstrom, '49, for further 

 discussion). 



— m 



■ps 



Fig. 64. Diagrams of part of surface of the sea urchin egg, illustrating (A) the different layers of the un- 

 fertilized egg and (fi to E) steps in the formation of the fertilization membrane, according to the views of 

 Runnstrom ('49). /, Gelatinous coat; vm, vitelline membrane; eg, cortical granules; c, cortex; e, endoplasm; 

 ps, plasma surface; m, fertilization membrane; hi, hyaline layer. (From Runnstrom, '49.) 



the eggs more susceptible to cross-fertiliza- 

 tion. The result implies that specificity is, 

 at least, partly controlled by the vitelline 

 membrane. 



In another species of animal, namely the 

 annelid Nereis, in which experiments on 

 removal of the vitelline membrane have been 

 performed, the denuded eggs do not fertilize 

 (Costello, '45, '49). In this species the vitel- 

 line membrane is a thick conspicuous struc- 

 ture through which a large amount of 

 gelatinous material is exuded after fertiliza- 

 tion. When the unfertilized eggs are placed 

 in an alkaline saline solution the jelly swells 

 but does not pass through the membrane, 

 which thereupon stretches and bursts. If 

 the treatment is delayed until shortly after 

 fertilization the membrane can be removed 

 without preventing the attached sperm from 

 entering the egg and without interfering with 

 normal development. 



Changes in Shape and Volume. In most 

 species of animals there are no marked 



Changes in Viscosity and Protein Solubility. 



Along with work on viscosity changes in 

 various kinds of cells, Heilbrunn ('28, '43) 

 and his students have supplied some data 

 concerning fertilization. In general fertiliza- 

 tion results in a marked increase, which 

 Heilbrunn interprets on the basis of his 

 calcium-release, protoplasmic gelation, the- 

 ory of activation. Mirsky ('36) reported that 

 the amount of protein that is soluble in 1 M 

 potassium chloride decreases upon fertiliza- 

 tion in sea urchins. This potassium chloride- 

 soluble protein forms highly viscous solutions \ 

 which show double refraction of flow and has, \ 

 therefore, been considered a structural pro- 

 tein like myosin. Connors and Scheer ('47) 

 find it to be electrophoretically homogeneous, 

 but, unlike myosin preparations, it possesses 

 no adenosine triphosphatase activity. It is 

 not, as yet, clear how the change in solubility 

 of this material may be correlated with 

 viscosity changes of the intact egg or with the 

 decrease in viscosity upon fertilization re- 



