354 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



E. Mechanisms of intrinsic morphochoresis. A discussion 



At the end of this second section, it is perhaps fair to contrast the two main 

 theoretical positions preferred by embryologists today. These concepts are not 

 exclusively based on the study of what we have called intrinsic morphochoresis, 

 but they necessarily include ideas relative to extrinsic morphochoresis, mainly 

 the processes of induction. 



For the first position we may refer to the "general conclusion" expressed by Ten Gate (1956, 

 his p. 84) as the result of a most clear and attentive introduction summarizing many 

 investigations concerning early development, and including one of the best accounts ever 

 given of the gradient field concept and its intricate implications; 



"a. First, it should be emphasized that nuclear, genetic factors have a predominant 

 influence on embryonic development. This appears to be demonstrated by the intensive 

 growth of the oocyte nucleus and by other phenomena. 



b. As demonstrated by many experiments, a restricted portion, the grey crescent, 

 appears to be highly important for the development of the entire embryo; even entodermal 

 differentiations were absent in ventral halves. Although some experiments show deviations 

 from this rule, it may be assumed that in this stage, the morphogenetic factors are mainly 

 concentrated in the grey crescent where all kinds of cytoplasmic constituents, including 

 the residual nuclear substance, appear to be present or at least in the near vicinity. 



c. In the subsequent stage, a distribution of morphogenetic factors throughout the 

 embryo appears to occur, whereby some of them become demonstrable in wide regions ; 

 consequently, these factors become highly mixed with each other. Perhaps, during this 

 period of expansion, the factors may be transferred from one cellular complex to an 

 adjacent one by means of induction. 



In terms of embryonic fields, i.e. self-organizing systems of morphogenetic factors, it may 

 be stated that in this period the fields are extending over increasingly larger regions and 

 therefore will largely overlap each other. The spreading of morphogenetic factors is 

 somewhat similar to the extensive cytoplasmic displacements which are so clearly visible 

 in ascidian eggs and by which large complexes become segregated. 



In the amphibian embryo, such displacements may be less extensive and definitive, 

 as is shown by the longer-lasting possibility of extensive regulations. Therefore, the effect 

 of gastrulation might be more important than in those species where a mosaic stage of 

 differentiation is reached as early as the late cleavage stages (mosaic types). 



d. The following period may be characterized by segregation of morphogenetic factors, 

 leading to their concentration in organogenetic areas. Segregation may be accompanied 

 by induction phenomena if the factors make their way, not only through intimately 

 coherent cellular complexes, but also from one complex (germ-layer or organ-part) to 

 another, partly via cytoplasmic bridges which are established during a certain period 

 of time. For example, in this stage, the prosencephalic factors residing in the early dorsal 

 blastopore-lip, after invagination may be transferred to the overlying pres. neural ectoderm 

 which itself already contains some of these factors also derived from the grey crescent and 

 arrived there through the continuous layer of micromeres of the blastula or the ectoderm 

 of the gastrula. Now, the embryonic fields come into the fore-ground; these build up 

 stronger potencies to develop a specific part by concentrating the available factors and by 

 reproducing them in a larger quantity. 



e. As a result of the activity of the fields, in the subsequent period, the mosaic stage of 

 differentiation is reached in which the embryo contains the segregated organogenetic areas. 



The effects of negative and positive affinity might underly the whole complex of 

 occurrences; spreading might be either a simple diffusion of factors into the remainder of 

 the egg or be stimulated by positive affinities of homologous and by negative affinities of 

 heterologous factors. 



These properties might be expressed in the general cytoplasmic displacements in the egg as 

 well as in the movements of cell-complexes and cells in subsequent stages of development." 



