318 



Embryogenesis: Progressive Differentiation 



stance, whose structvire is therefore not 

 essentially disturbed by centrifuging. The 

 possibility that this architecture of the 

 ground substance might involve liquid crys- 

 tals has been discussed (Needham, '42; Har- 

 rison, '45). Conklin ('24) stated that the 

 organization of the egg resides in the internal 

 framework of entoplasm and the cortical 

 layer of ectoplasm. 



EVIDENCE THAT THE STRUCTURAL 



BASIS OF EGG ORGANIZATION RESIDES 



IN THE EGG CORTEX 



If some permanent internal framework 

 exists in the entoplasm of the ground sub- 

 stance, it should be possible to obtain evi- 

 dence of its existence from viscosity measure- 

 ments (Howard, '32). The presence of 

 cohering molecular aggregates ramifying 

 through a liquid phase would be expected to 

 cause plastic flow of the system as a whole, 

 as a result of a certain amount of mechanical 

 rigidity to shear which is characteristic of 

 such structiires. Centrifugation experiments 

 on two sea lu-chins {Arbacia punctulata and 

 Strongylocentrotus purpuratus) revealed no 

 imequivocal plasticity and led Howard to 

 conclude (p. 368) that ". . . no continuous 

 structure is present which can significantly 

 affect diffusion within the resting egg." If 

 so, there is no constant protoplasmic structure 

 in the entoplasm which could account for 

 the relatively stable organization of the egg. 

 It has been noted that usually the ectoplasm 

 (cortex) of the egg is relatively unaffected by 

 centrifugation experiments, and it was nat- 

 ural to suggest that the ultrastructural basis 

 for egg organization must be localized in 

 the egg cortex (Motomura, '35; Raven and 

 Bretschneider, '42; and others). The impor- 

 tance of the ectoplasm in this respect would 

 also seem to be indicated by the work of 

 Horstadius, Lorch and Danielli ('50), who 

 withdrew large quantities of entoplasm from 

 the sea urchin egg (reducing the volume of 

 the egg by more than 50 per cent) and still 

 obtained normally proportioned plutei; never- 

 theless, these authors seem reluctant to sug- 

 gest that their experiments provide any 

 conclusive evidence on the problem. 



If the basic polar organization of the egg 

 does reside in the cortex, it would be essential 

 that at least some components of the cortex 

 maintain their relative positions throughout 

 cleavage; otherwise cleavage would disrupt 

 such an organization. Runnstrom ('28a, '28b) 

 has described a dark-field ring in the cortex 

 of the vegetal half of the sea urchin egg 



which maintains its position unchanged 

 throughout cleavage; during gastrulation the 

 cells in which this dark-field ring is localized 

 largely become invaginated to form the walls 

 of the archenteron. Accordingly, part of the 

 cortex, at least, seems not to be dislocated 

 during formation of cleavage furrows and 

 could serve as the locus of the basic organi- 

 zation of the egg along the animal-vegetal 

 axis (see Lehmann, '45). 



In general it may be said that the axis of 

 differentiation rarely becomes dissociated 

 from the original axis of polarity; conse- 

 quently it is of basic importance for an un- 

 derstanding of differentiation that we arrive 

 at some concept of the ultrastructure of the 

 egg responsible for the basic polarity made 

 visible by morphological and physiological 

 differences. Recent speculations by Weiss 

 ('49a, '49b, '50) in terms of molecular ecol- 

 ogy provide a model of the way in which 

 an initial organization of the egg crust could 

 impose organization upon the more fluid egg 

 core. 



SPECIFICATION OF BLASTOMERES 



ACCORDING TO THE PORTION OF THE 



CYTOPLASM SEGREGATED WITHIN THEM 



The most striking feature about the de- 

 scriptive embryology of certain annelids, 

 mollusks, ctenophores and tunicates is the 

 precocious localization of visibly different 

 cytoplasmic areas and the segregation of 

 these areas into specific blastomeres or groups 

 of blastomeres. In many instances isolation 

 and transplantation experiments prove that 

 such blastomeres or groups of blastomeres 

 differ markedly from other blastomeres in 

 their developmental capacities. 



ANNELIDS AND MOLLUSKS 



Experiments on Eggs Lacking Polar Lobes 

 and Pole Plasms. Especially clear-cut evi- 

 dence for specification of blastomeres accord- 

 ing to the portion of the cytoplasm segre- 

 gated within them comes from experiments 

 on the eggs of the mollusk Patella (Wilson, 

 '04b) and the annelid Nereis (Costello, '45). 

 The trochophore larva of Patella is illustrated 

 in Figure 120^. If a single cell of the first 

 quartette of micromeres is isolated (Fig. 

 1205), it develops into a partial larva con- 

 sisting of four primary trochoblasts, two 

 smaller secondary trochoblasts, two apical 

 tuft cells, and a group of non-ciliated ecto- 

 blast cells (Fig. 120C). The first quartette of 

 micromeres divides into four upper cells 



