Selected Invertebrates 



323 



white photographs of living eggs (Conklin, 

 '05b).* 



Do differences in developmental capacities 

 of blastomeres parallel the visible differences? 

 Conklin ('05c, '06) sought an answer to this 

 question by defect experiments. Eggs in the 

 2-, 4-, 8-cell stages, etc., contained within the 

 chorion, were strongly spurted with a pipette 

 or were shaken in a vial, whereby some of 

 the blastomeres were injured and others re- 

 mained uninjured and continued to develop. 

 Injured blastomeres were rarely killed, but 

 their nuclei were frequently broken and 

 their chromosomes scattered, such that these 

 cells could not undergo cleavage. By such 

 crude treatment, eggs were obtained with 

 injured and uninjured blastomeres in vari- 

 ous combinations. These could be sorted 

 into groups as follows: eggs with the right 

 half uninjured, the left half injured, and 

 vice versa; those with the anterior half unin- 

 jured and the posterior half injured, and vice 

 versa. In this way Conklin obtained sufficient 

 material to study in detail the development 

 of right and left half embryos, three-quarter 

 embryos, anterior and posterior half embryos, 

 quarter embryos and eighth or sixteenth em- 

 bryos. Regulation of the fragments was 

 limited entirely to closure of the larva (or 

 organ) on the injured side and there was no 

 restitution of missing parts. For example, the 

 anterior half-egg gave rise to an embryo con- 

 sisting of ectoderm, neural plate, sense spots, 

 notochord and entoderm, but lacking muscles 

 and mesenchyme; the posterior half-egg gave 

 rise to an embryo consisting of ectoderm, 

 mviscles (at least myoblasts), mesenchyme 

 and entoderm, but lacking neural plate, sense 

 spots and notochord. Each fragment of the 

 egg under these circumstances gave rise to 

 those structures which it would have formed 

 as part of the whole developing egg (see 

 Fig. 121). From results of this sort Conklin 

 concluded that development of the ascidian 



* No such elaborate color scheme exists in other 

 tunicate eggs, although Berrill ('29) reports an 

 orange substance in the eggs of Boltenia hirsuta 

 which segregates into the muscle and mesenchyme 

 cells exactly as does the yellow substance in Styela. 

 In Ciona intestinalis, Phallusia mamillata and 

 Molgula manhattensis the eggs are not colored; 

 nevertheless the same ooplasmic regions can be 

 recognized in stained sections, and have the same 

 prospective fates. Various ooplasmic regions in yet 

 another ascidian egg {Ascidiella aspersa) have the 

 same prospective fates, although the fates of these 

 regions can be demonstrated only with the aid of 

 vital stains (Tung, '32). In Ascidiella scabra slight 

 differences in the position of organ-forming regions 

 have been demonstrated (Dalcq, '38) . 



egg is a mosaic work since the individual 

 blastomeres or groups of blastomeres are 

 composed of different kinds of ooplasmic 

 materials, and moreover since the develop- 

 mental fate of any ascidian blastomere or 

 group of blastomeres is primarily a function 

 of its material content. He saw no indication 

 of dependent differentiations or inductions. 

 He reached similar conclusions following 

 defect experiments on the eggs of the Euro- 

 pean tunicate, Phallusia mamillata (Conk- 

 lin, '11). His conclusions received strong 

 support from the studies of von Ubisch ('39a) 

 who removed blastomeres at later stages (32- 

 and 64-cell stages) when each kind of 

 ooplasm was restricted to separate blasto- 

 meres. Systematically removing each group 

 of blastomeres, he found only one instance 

 where the potency of the remaining cells 

 was greater than their prospective fate, viz., 

 ectoblast could give rise to entoderm.f 



DIFFERENTIATION WITHOUT 

 CLEAVAGE 



In annelids, mollusks, ctenophores and 

 tunicates progressive differentiation (i.e., 

 temporal and spatial restriction of potencies) 

 seems to involve primarily a precocious lo- 

 calization of visibly different cytoplasmic 

 areas and the segregation of these areas into 

 specific blastomeres or groups of blasto- 

 meres whose developmental capacities then 

 prove to differ markedly from other blasto- 

 meres. To what extent can differentiation 

 occur if cleavage fails to segregate these 

 localized areas? Lillie ('02) was able to 

 suppress cleavage by treating unfertilized 

 or fertilized Chaetopterus eggs with sea 

 water containing potassium chloride. His 

 most striking illustration of the degree to 

 which differentiation can progress without 

 cleavage is illustrated in Figure 122^ 

 (which should be compared with the normal 

 trochophore larva of Chaetopterus illustrated 

 in Fig. 122S). His description reads as fol- 

 lows (p. 481): "This structure possesses a 

 certain undeniable resemblance to a trocho- 

 phore: if the smaller hemisphere be com- 



■\ Isolation experiments (Berrill, '32; Tvmg, '34; 

 Cohen and Berrill, '^Q; Rose, '39; von Ubisch, '40; 

 Reverberi and Minganti, '46a,b; Pisano, '49) gave 

 essentially similar results. There was some disagree- 

 ment as to whether cerebral vesicles could form in 

 isolated animal halves, although there was agree- 

 ment that sensory spots could not form in such 

 halves; similarly there was disagreement as to 

 whether entoblast could give rise to ectoderm in iso- 

 lated vegetal halves. 



