220 



Embryogenesis: Preparatory Phases 



segregation to form the posterior yellow 

 crescent. Opposite this region, the light gray 

 crescent arises at the future anterior region 

 of the egg. As a result, the animal hemi- 

 sphere is occupied by clear protoplasm, the 

 remainder of the vegetal hemisphere (except 

 for the two above-mentioned crescents) by 

 dark gray yolk. 



The formation of the gray crescent in the 

 frog egg, which follows fertilization and is 

 correlated in at least a high percentage of 

 cases with the position of the median plane 

 of the embryo, may be considered a segrega- 

 tion phenomenon analogous to crescent for- 

 mation in the egg of Styela. 



It appears possible, by studying a series 

 of different egg species, to establish a cor- 

 relation between the time of ooplasmic 

 segregation and the degree of embryonic 

 determination. Invertebrate eggs of the vari- 

 ous animal groups can be divided into dif- 

 ferent categories as regards the time of 

 onset, and the pattern, of ooplasmic segre- 

 gation. It is obvious that there are different 

 relationships between the pattern of segre- 

 gation and the cleavage pattern in these 

 different forms. An isolated blastomere of 

 an egg cleaving differentially at the first 

 cleavage (such as Chaetopterus, Dentalium, 

 Nereis or Sabellaria) would be expected to 

 develop as a partial embryo because the 

 segregated material is different in the two 

 daughter blastomeres. 



The apparent paradox between the correla- 

 tion of ooplasmic segregation with embryonic 

 differentiation in isolated blastomeres and 

 egg fragments, as compared with the lack 

 of correlation between the sedimentation of 

 visible particles and embryonic differentia- 

 tion, in the case of centrifuged eggs, can 

 be resolved theoretically as follows: We 

 need only assume that the mechanism of 

 normal ooplasmic segregation segregates both 

 visible formed inclusions and invisible histo- 

 genetic substances. The invisible histogenetic 

 substances could be presumed to become 

 associated with the hyaline protoplasmic 

 materials which occupy the interstices (as 

 ground substance) between the granules or 

 vacuoles. Centrifuging might easily displace 

 the larger visible particles and vacuoles with- 

 out displacing the invisible substances (of 

 molecular dimensions or somewhat larger) 

 associated with the hyaline protoplasm 

 (protein framework?) of the cell. This con- 

 clusion is basically that reached by Conklin 

 ('31), from his experiments centrifuging the 

 Styela egg. It is not the visible granules that 

 have histogenetic value, but the special lo- 



calized hyaline protoplasm of the areas with 

 which these granules are normally asso- 

 ciated. 



If this view be accepted, it becomes of 

 paramount importance to work out the 

 mechanism by which ooplasmic segregation 

 is brought about. This mechanism must be 

 able to accomplish both a visible stratifica- 

 tion of the suspended ooplasmic substances 

 and a parallel, but invisible, segregation of 

 the so-called "formative stuffs." 



Eggs of animal species whose cleavage 

 characteristics are known can be aligned in 

 a series with the most determinate (cteno- 

 phores, rotifers, and nematodes) at one ex- 

 treme, the least determinate at the other, and 

 with all degrees of gradation between the 

 two. When this is done, it appears that the 

 most rigidly "determinate" cleavage types 

 are found in the forms which represent the 

 most markedly "mosaic" pattern of differen- 

 tiation. Nevertheless, the chief differences 

 between the so-called "mosaic" and "regula- 

 tive" eggs are merely (1) a different morpho- 

 logical relation of the cleavage pattern to 

 certain prelocalized or segregated embryonic 

 areas (i.e., unequal as contrasted with equal 

 distribution of these segregated histogenetic 

 areas between the blastomeres at a given 

 cleavage), (2) different time relationships 

 between the determination of embryonic 

 areas and such developmental events as fer- 

 tilization, cleavage, and gastrulation, and 

 (3) the regulative (and regenerative) ca- 

 pacity of the developing embryo, including 

 the degree of interaction of the parts involv- 

 ing determination by diffusible substances. 

 It appears that even the most "mosaic" egg 

 has some integrative or regulative powers, 

 which act during normal development as 

 well as under special experimental condi- 

 tions. 



FUNCTION OF PARTITION MEMBRANES IN 

 CELLULAR DIFFERENTIATION 



Differentiation without cleavage was first 

 adequately demonstrated by the studies of 

 Lillie ('02, '06) on the eggs of Chaetopterus. 

 Treatment of unfertilized eggs or of un- 

 cleaved fertilized eggs with sea water con- 

 taining a small additional amount (0.25 

 per cent) of potassium chloride resulted in 

 the development of these into ciliated struc- 

 tures with segregated internal contents. 

 These ciliated "embryos" resembled annelid 

 trochophores in a number of respects. 

 Brachet ('37), among others, has repeated 

 Lillie's observations, and demonstrated that 



