11 INTRINSIC MORPHOCHORESIS MECHANISMS 355 



It is unnecessary to describe the alternative position at length, since it has 

 induced the preceding sections. The difference lies first in a more accurate 

 consideration of the cytoplasmic organization in the oocyte and of its acquisitions; 

 second, in a more precise appreciation of the properties of the grey crescent, 

 which are deduced from definite observations and experiments, and not from 

 the abusive projection back on to grey crescent material of the activities recognized 

 in the organizer; third, in a more strict use of the gradient field concept, which 

 is only licit when positive arguments permit, that is in the cortex of the 

 fertilized egg and the cleavage stages, in the mesoblastic layer of the archenteric 

 roof, in the induced neural plate, and (perhaps) around the presumptive areas of 

 some secondary organs; fourth, in a reluctance to admit that induction processes 

 exist in cases where they are not actually demonstrable, as in the grey crescent 

 region or along the epithelial layers; and fifth, in the recognition that regional 

 differences are responsible for what has been described above as morphogenetic 

 potential(s). 



To be quite clear, it will be useful to submit the above quotation to further 

 criticism; this will be done without any intention to depreciate Ten Gate's highly 

 valuable contribution. The discussion will concern only the conclusion which a 

 most skillful and competent worker has felt authorized to draw from an extensive 

 revision of the embryological literature. 



To item a. This general statement must be admitted. For § 2, the intensive growth 

 of the oocyte nucleus is certainly important from several points of view, but whether or 

 not it expresses a special intervention of genetic factors, we do not know. The fluid content 

 of the germinal vesicle is not necessarily different from that of any somatic nucleus. 



To item b. Although one can agree with the general sense of this statement, the grey 

 crescent region is not as priviledged as presented by Ten Gate. Absence of differentiation 

 in isolated ventral halves is a classical assertion which, as was shown by DoUander (1950), 

 lacks definite proof. Careful experiments in which ligature was combined with vital 

 staining have demonstrated that the ventral halves of Triton helveticus eggs are able in a 

 large proportion of cases to form an embryo, if they are not smaller (preferably a bit 

 larger) than the dorsal halves. Holtfreter (1951 his p. 119, note i) has attempted to invali- 

 date Dollander's experiment with the objection that his ligatures were not really frontal. 

 Although this criticism was a priori met in Dollander's original research, he nevertheless 

 made new vital staining experiments which showed that no error could have been com- 

 mitted (1952). Accordingly, the opinion that "morphogenetic factors are mainly con- 

 centrated in the grey crescent" is consistent neither with the regulation of isolated 

 ventral halves, nor with the production of new blastoporal lips in inverted eggs (p. 343). 

 Whether or not the "residual nuclear substance" plays a role, seems impossible to decide 

 owing to lack of experimental possibilities to check it. 



To item c. The redistribution of morphogenetic factors and their secondary mixing 

 is an assumption resulting from the two exclusive roles attributed to the grey crescent 

 region. If we are concerned with pregastrula stages, it is quite unnecessary to imagine 

 active inductions from one blastomere to another. Of course, for the sea-urchin egg, such 

 processes do seem to occur (p. 350) but only under experimental conditions. It has been 

 claimed that in ascidian eggs, the induction of the neural territory takes place during 

 cleavage (Vandebroek, 1938; Rose, 1939; Reverberi and Minganti, 1949); without 

 questioning the accuracy of the experimental data, I suspect that this induction is of a 

 special kind and need not to be discussed here. 



To item c § 2. As far as pregastrula stages are concerned — and the allusion to dis- 

 placements of cytoplasm in Ascidians indicates that they are — the assumption of several 

 overlapping fields is invalid, since it is based only upon the erroneous "expansion of 



Literature p. 483 



