326 



Embryogenesis: Progressive Differentiation 



react with one another, but give rise to a 

 separate individual. This would seem to im- 

 ply that they do interact with one another 

 under normal circumstances. 



Timg ('34) has accomplished the fusion 

 of two tvmicate eggs in the 2-cell stage, ob- 

 taining results which favor rigid mosaic 

 development in that two complete sets of 

 organs formed. However, von Ubisch ('38, 

 especially '39a) was able to produce giant 

 tadpole larvae from fusion of two tunicate 

 eggs oriented in such a way that their ani- 

 mal-vegetal axes were parallel. It appeared 

 that a point-to-point fusion of identical cyto- 

 plasmic areas occvured, with each such area 

 then forming an organ larger than normal. 

 Thus no evidence for interaction of blasto- 

 meres was furnished by either investigator. 

 Nevertheless there is evidence from other 

 experiments for the induction of at least 

 two structures in the tadpole larva of tuni- 

 cates, viz., adhesive papillae and sensory 

 cells. Tung ('34) separated the animal half 

 from the vegetal half in the 8-cell stage and 

 rotated the former through 90 to 180 degrees 

 before replacing it. Adhesive papillae were 

 frequently absent following such rotation; 

 when present they always occurred at the 

 original anterior extremity of the vegetal 

 half, although they originate from cells of 

 the animal half. This means that they have 

 formed from ectoderm which does not nor- 

 mally produce them and that the stimulus 

 for their formation originated in the an- 

 terior vegetal cells. This appears to be an 

 example of true induction. Similarly in cen- 

 trifugation experiments in which variovis 

 structures underwent relatively normal his- 

 togenesis, but occurred in abnormal rela- 

 tionships to one another, Tung, Ku, and 

 Tung ('41) observed that adhesive papillae 

 differentiated only from ectoderm which was 

 in contact with entoderm (no exception in 

 49 cases). Reverberi and Minganti ('51) 

 agree that the stimulus necessary for de- 

 velopment of adhesive papillae emanates 

 from anterior vegetal cells, but they find, in 

 contrast to Tung ('34), that such adhesive 

 papillae can be stimulated to form only from 

 anterior animal cells, i.e., from cells which 

 normally form adhesive papillae. They pre- 

 fer, therefore, to speak of the stimulus as 

 an "evocation" rather than as an "induc- 

 tion." 



Similar evidence is available that inter- 

 action of blastomeres is involved in the dif- 

 ferentiation of sensory cells. Isolated lateral 

 halves of the 8-cell stage form sensory cells, 

 as do isolated anterior halves; by contrast, 



isolated posterior halves or isolated animal 

 and vegetal halves do not form sensory cells 

 (Berrill, '32; Tung, '34; von Ubisch, '40). 

 Such cells originate from the animal half, 

 but it appears that some stimulus from the 

 anterovegetal half is necessary for their dif- 

 ferentiation. This conclusion is supported by 

 Rose ('39), who found that cerebral vesicles 

 and sensory cells formed in 15 of 46 cases 

 when the two posteroanimal cells were com- 

 bined with the two anterovegetal cells, 

 whereas in reciprocal combinations (two 

 anteroanimal plus two posterovegetal cells) 

 neither cerebral vesicles nor sensory cells 

 formed in 20 attempts.* Rose had some evi- 

 dence that the inductor of the sensory cells 

 was restricted to the prospective entodermal 

 portion of the two anterovegetal cells; how- 

 ever, von Ubisch ('40) obtained sensory 

 cells in defect experiments involving the 

 complete removal of prospective entoderm. 

 Reverberi and Minganti ('51) agree that 

 some stimulus necessaiy for development 

 of brain and sensory cells emanates from 

 the anterior vegetal cells; but, as in the case 

 of adhesive papillae, they believe that the 

 development of such structures can be stimu- 

 lated only from cells which usually give 

 rise to them, i.e., only from anterior animal 

 cells. Again, then, they prefer to speak of 

 an "evocation" of brain and sensory cells, 

 rather than of their "induction." They are 

 convinced that this evocation is mediated 

 by a chemical substance. 



In the words of Reverberi and Minganti 

 ('51), "all other presumptive territories (of 

 muscle cells, notochord, epidermis, mesen- 

 chyme, endoderm) present a complete ca- 

 pacity of self-differentiation, even in the ab- 

 sence of this 'center,' that is of the anterior 

 vegetal blastomeres. "f 



* But there must be species differences in this 

 respect, since Tung ('34) obtained differentiation 

 of sensory cells when he rotated the animal half of 

 the 8-cell stage through 180 degrees which would 

 place the anterior animal cells above the two 

 posterior vegetal cells. 



f Both Rose ('39) and von Ubisch ('40) noted that 

 myoblasts complete their differentiation into muscle 

 cells only when the anterior vegetal cells are pres- 

 ent together with the posterior vegetal cells from 

 which myoblasts originate. However, it seems 

 likely that this stimulus necessary for complete dif- 

 ferentiation of myoblasts may be simply a mechan- 

 ical one imposed normally by the elongating chorda 

 which originates from cells of the anterovegetal 

 region, and of quite a different nature from the in- 

 ductive stimulus necessary for formation of ad- 

 hesive papillae and possibly of sensory cells. For 

 more recent experiments see Reverberi and Min- 

 ganti ('47). 



