Selected Invertebrates 



325 



of blastomeres. This is in distinct contrast 

 to identical combinations of sea urchin 

 blastomeres, as noted below. Each set of 

 blastomeres simply self-differentiated regard- 

 less of the other blastomeres present si- 

 multaneously. Novikoff ('38) combined 

 halves and quarters of the egg of the annelid 

 Sabellaria with another whole egg. The addi- 

 tion of these extra egg fragments gave no 

 inductions in the host; any added fragment 

 simply self-differentiated into those struc- 

 tures characteristic of its prospective fate 



lation is possible in the eggs of annelids and 

 mollusks imder special conditions. It was 

 noted earlier that in the absence of the nor- 

 mal source of ectodermal structures in the 

 Tubifex egg, such structures could arise from 

 cells originally believed to be specified for 

 mesodermal differentiation only (Penners, 

 '38). Also it was noted that in the absence 

 of mesodermal structures metamerism was 

 lacking and ectodermal and entodermal 

 structures were displaced, suggesting an or- 

 ganizing influence of the mesoderm upon the 



Fig. 123. Types of twins experimentally produced in annelids. A, Duplicitas cruciata twin of Chaetopterus 

 (from Titlebaum, '28). B. Duplicitas cruciata twin of Sabellaria (from Novikoff, '40). C, Duplicitas cruciata 

 twin of Nereis (from Tyler, '30). 



(i.e., an extra A or B cell added extra apical 

 cilia, an extra C cell added an extra apical 

 tuft, and an extra D cell added an extra 

 post-trochal region, etc.). The localization of 

 cytoplasmic factors for development of apical 

 tuft and post-trochal region in the polar 

 lobes of annelids and mollusks has already 

 been discussed; attempts by Novikoff to 

 transplant polar lobes to entire eggs of Sabel- 

 laria were unsuccessful in that nothing 

 additional formed in the hosts to which such 

 polar lobes were attached (although the 

 union of polar lobes to eggs was sufficiently 

 close that vital stain passed from the former 

 into the latter). Fusion of two or more 

 eggs (Hatt, '31; Novikoff, '38) did not re- 

 sult in a single giant larva of unit structure 

 but yielded double embryos or more complex 

 monsters, each egg in the fused mass dif- 

 ferentiating independently of the others. 



In spite of these negative results insofar 

 as demonstrating interaction of blastomeres 

 is concerned, a considerable amount of regu- 



distribution of ectodermal and entodermal 

 derivatives. Moreover, in Tubifex an isolated 

 CD or D cell containing the pole plasm regu- 

 lates itself into a normally proportioned, 

 though small, worm. But even more striking 

 is the observation that if the vegetative pole 

 plasm be equally distributed to the first two 

 blastomeres (Tubifex, by heat or depriva- 

 tion of oxygen, Penners, '24b; Chaetop- 

 terus, Nereis and Cumingia by compression, 

 high or low temperatures, centrifugation or 

 anaerobiosis, Titlebaum, '28, Tyler, '30; 

 Sabellaria by treatment with potassium 

 chloride, Novikoff, '40), double monsters of 

 the duplicitas cruciata type are formed (Fig. 

 123), or the blastomeres may, if isolated, give 

 rise to more or less complete larvae. Costello 

 ('45) has emphasized that the treatment 

 must do more than distribute pole plasm 

 equally to the first two blastomeres; it 

 must set up an effective barrier between 

 the two cells such that each of the first two 

 blastomeres and their derivatives do not 



