222 



Embryogenesis: Preparatory Phases 



ing a sheet. This confining membrane, the 

 hyaline plasma layer, in the forms where it 

 exists, is therefore necessary for the forma- 

 tion of a normal blastula. 



Removal of the egg membranes and treat- 

 ment with calcium-low sea water reveals 

 that the blastomeres clinging together in 

 loose clusters are attached to one another by 

 fine strands, plasmodesms or cell bridges. 



Fig. 72. Figures to illustrate the movement of the 

 intra-hyaloplasmic fluid. A, Condition of the hya- 

 line layer during the monaster and the streak 

 stages. The amount of the fluid is small. B, Within 

 three minutes prior to cleavage, fluid which comes 

 from outside through the hyaline layer acciunu- 

 lates between the layer and the cell surface. The hy- 

 aloplasmic envelope is thus inflated and conse- 

 quently the attachment fibers are stretched. C, After 

 cleavage, the fluid passes into the space between the 

 freely movable apposed surfaces, releasing the ten- 

 sion on the attachment fibers. Hy, Hyaline layer; 

 sp, fluid space; cyt, cell body. (From Dan and Ono, 

 '52.) 



These fine processes may help to prevent 

 shifting of the relative positions of the blasto- 

 meres, but are to be considered as secondary 

 cell-connectives. The primary connection be- 

 tween blastomeres is the spindle remnant 

 (stalk). At certain stages after a given divi- 

 sion the spindle remnant is sufficiently 

 tough so that it must be cut across in order 

 to separate blastomeres of certain eggs for 

 isolation studies (Plough, '27; Costello, '45). 

 Such spindle remnants may persist from 

 previous cleavages in such eggs as those of 



Dendraster (Moore, '45), where three spin- 

 dle remnants may be clearly demonstrated at 

 the four-cell stage. One of these spindle 

 remnants resulted from the first cleavage; 

 two from the second. The conditions under 

 which the primary or secondary cell con- 

 nectives normally disappear, or the extent 

 to which they continue to persist in later 

 development, are largely unknown. Dan 

 ('52) does not find long-persistent spindle 

 remnants in sea urchin eggs. It is obvious 

 that rigid cell-connectives are not present 

 on blastomeres engaged in such morpho- 

 genetic movements as the formation of pri- 

 mary mesenchyme in the sea urchin egg or 

 the cytotaxis accompanying gastrulation in 

 amphibian eggs. However, in forms which 

 completely lack a hyaline plasma layer 

 (such as certain coelenterate eggs) the pro- 

 toplasmic coimectives (spindle remnants, 

 etc.) and the intercellular cement may be 

 the only factors maintaining the shape of 

 the morula or planula. 



Dan ('52) has suggested a very interesting 

 interpretation of blastulation in the echino- 

 derm egg. He begins with the assumption 

 that the fine processes of the egg surface 

 (described above) attach inside the hyaline 

 plasma layer. The apparent increase in 

 thickness of the hyaline plasma layer just 

 before cleavage is not, according to Dan, 

 a thickening in the usual sense of the word. 

 It is due to the accumulation of a fluid 

 between the egg surface and the layer. This 

 is in agreement with Gray's ('24) suggestion 

 that the hyaline layer has a hard outer re- 

 gion and a fluid inner zone. Although the 

 egg diameter remains constant, the diameter 

 of the hyaloplasmic envelope does increase 

 before the cleavage furrow forms. The fine 

 protoplasmic processes, although stretched 

 considerably, are still attached to the inner 

 surface of the hyaline layer. This sudden 

 accumulation of fluid occurs immediately 

 preceding each cleavage, and then the fluid 

 passes to the region between the blasto- 

 meres (see Fig. 72). At each successive cleav- 

 age, more fluid accumulates as the hyaline 

 layer is distended, but since the blastomeres 

 are fastened to the hyaline layer by the 

 protoplasmic processes, they tend to be 

 drawn out toward it and the fluid continues 

 to move toward the center to form the blasto- 

 coele. Dan considers that the fluid is a 

 secretion of a water-accumulating colloidal 

 substance, set free in the intrahyaline space. 

 When gum arabic is added to the surround- 

 ing sea water, the "thickening" of the hya- 

 line layer is suppressed and the resulting 



