218 



Embryogenesis: Preparatory Phases 



be penetrated by sperm and showed the 

 surface activation phenomena characteris- 

 tic of fertilization. Not one of these frag- 

 ments ever cleaved. Cytological examination 

 of sections of these fragments gave evidence 

 that sperm aster formation did not take 

 place. Attempts to activate non-nucleated 

 fragments by means of parthenogenetic 

 agents likewise produced surface activation 

 responses, but no cleavage. The conclusion 

 was reached that substances essential for 

 aster formation are released from the germi- 

 nal vesicle at the time of its rupture; in 

 the absence of this material, cleavage is not 

 possible. 



The report by A. R. Moore (quoted as a 

 personal communication by P. Weiss, '39) 

 that "cleavage substances" may be displaced 

 by centrifugal force in the egg of Dendraster, 

 and hence only one end of the egg can 

 respond to parthenogenetic agents, is of 

 considerable interest. Since Dendraster eggs 

 are fertilizable when fully mature, the theo- 

 retical implications are subject to the same 

 criticism as in the case of "parthenogenetic 

 merogony" of the Arbacia egg. This "cleav- 

 age substance" in Dendraster cannot be 

 completely analogous to the "cleavage sub- 

 stance" described by Beams and King ('37) 

 on the basis of ultra centrifugation experi- 

 ments on fertilized Ascaris eggs. 



REDISTRIBUTION OF EGG SUBSTANCES 



AND THEIR RELATION TO CLEAVAGE 



PATTERNS 



Streaming movements of the cytoplasm, 

 and especially those taking place during 

 certain phases of mitosis, are well known 

 from the observations of Spek ('18, '26), 

 Conklin ('38), and many others. Whether 

 these movements can be due to localized 

 changes in surface tension at the egg surface 

 as once suggested is now highly problemati- 

 cal. The measurements of surface behavior 

 of the eggs of several different marine ani- 

 mals by the kaolin particle method of Dan 

 and his co-workers ('37) now give exact 

 data which appear to preclude such an in- 

 terpretation. The low values for total tension 

 (0.2 dyne per centimeter for the Arbacia 

 egg) obtained by E. N. Harvey ('31), Cole 

 ('32), and Cole and Michaelis ('32) like- 

 wise suggest that surface tension cannot be 

 of significance in such cellular activities. 

 With such a low initial tension, any changes 

 due to surface tension-lowering substances 

 would involve negligible amounts of surface 

 force. The flowing movements may therefore 



be due to the activity of asters (or centri- 

 oles?) or to some other (and unknown) 

 cause. 



Over and above such streaming movements 

 taking place during the process of cell divi- 

 sion are the extensive redistributions of 

 ooplasmic substances which occur in various 

 eggs at different times during maturation, 

 before or after fertilization, or during early 

 cleavage. In the egg of the sea urchin, 

 Arbacia, which is practically homolecithal 

 when fully mature, an outwardly directed, 

 radial migration of the echinochrome pig- 

 ment granules follows fertilization, so that 

 these granules come to lie in the gelated 

 cortical region (Wilson, '26). The cortical 

 gelation itself is another consequence of 

 the chain of reactions inaugurated by fer- 

 tilization. This visible radial segregation is 

 typical of many echinoderm eggs, and per- 

 haps correlated with the differentiation 

 equipotentiality of the first four blastomeres 

 when separated or isolated. An exception, 

 among echinoderms, to this simple corti- 

 calward segregation may perhaps be found 

 in the case of Paracentrotus eggs. The eggs 

 of some individuals of Paracentrotus lividus 

 are characterized by a certain amount of 

 polar differentiation in the form of a sub- 

 equatorial pigment band (Boveri, '01). This 

 band has been used (Horstadius, '28, '39) 

 as a means of orienting the egg for cutting 

 animal or vegetal fragments. It is not a gen- 

 eral characteristic of sea urchin eggs, but the 

 accumulation of pigment in this zone may 

 be considered another form of ooplasmic 

 segregation. 



The more classic examples of animal-vege- 

 tal ooplasmic segregation are to be found 

 in eggs showing the so-called "determinate" 

 type of cleavage. Since there is an original 

 polarity of the egg that can usually be traced 

 back to the earliest stages of the oogonium, 

 it is recognized that the simplest type of 

 ooplasmic segregation along the animal- 

 vegetal axis is essentially a repolarization 

 of certain of the egg constituents. Only this 

 repolarization can be considered to come 

 within the scope of Spek's ('30, '33, '34) term 

 "bipolar differentiation," as applied to the 

 phenomenon in several different egg species. 



As a typical example of ooplasmic segre- 

 gation, the process in the egg of the annelid. 

 Nereis, may be described. The egg of Nereis 

 before fertilization is flattened somewhat in 

 the polar axis and contains a large central 

 germinal vesicle (oocyte nucleus), around 

 which is arranged a double row of relatively 

 large oil droplets. The other cytoplasmic 



