II MORPHOCHORETIC PATTERN CLEAVAGE 34! 



turnover in the various territories. These questions have been partially solved for Para- 

 centrotus liv., where considerable differences have been detected between ectoblast, mesen- 

 chyme, endoblast (Lison and Pasteels, 195 1). In Lytechinus v. the measurements performed 

 by McMaster (1955) are only in partial agreement. Nevertheless, the results of this author 

 indicate that at the i6-cell stage, there are differences in the rate of DNA assimilation 

 according to the tiers of blastomeres. 



D. Kinetic and causative aspects of intrinsic morphochoresis 



In the intimacy of its events, mitosis is fundamentally a kinetic process, not only 

 concerning the dividing of chromosomes btit also the distribution of the various 

 cytoplasmic organelles. However, the prevailing usage is to speak only of kinetics 

 when the involved animal germs cease to be a spherical mass of cells in their 

 development toward a more complicated form, or when blastoderms of reptiles 

 or birds elongate along their main axis with the concomitant appearance of an 

 embryonic shape. At this critical stage, when the germ charges from its spherical 

 form, general and local forces are simultaneously at play. 



In the first part of this century, appropriate methods which revivalised experi- 

 mental embryology have revealed active deformation and characteristic shifting 

 of the various territories. These strictly ordinated and harmonious movements are 

 the core of morphochoresis. This kinetics is described in all modern books on 

 development^ We have only to consider here the general aspects and theoretical 

 interpretation of these characteristic events. 



{a) The factors at work 



Developmental kinetics is of course quite a progressive process, which begins within 

 the cells some time before becoming externally visible. One necessary condition for 

 its appearance is certainly a sufficient redtiction of the cell size; another prerequisite 

 is the existence of forces capable of overcoming the cohesion progressively es- 

 tablished between the cells. On the other hand, one important factor, although 

 antikinetic, is the existence of an extracellular covering layer. Embryologists of 

 the past century had already become acquainted with the hyaline layer of the sea- 

 urchin eggs. Much later the coat was discovered in amphibians' and masterfully 

 analysed by Holtfreter (1943). In recent years its existence was recognized in 

 birds (Malan, 1953) and in mammals (Dalcq, 1954c). The intrinsic mechanism of the 

 morphochoretic movements was so far only explained by some indirect but never- 

 theless interesting inferences. The tentative idea is analogous to that concerning 

 amiboid movements; that is, contractile proteins coupled with an energetic system 

 producing ATP^ and responsible for dephosphorylation {cf. Mulnard, 1956). 



It may be assumed that they are apparently due to a modification or perhaps 

 some difTuse contraction (Lewis, 1947), taking place in the non-coated part of 

 the cell. This change takes place almost simultaneously, (and with a similar and 

 definite character) in the cells of each presumptive territory which, at this very 



1 Recently a valuable contribution to the movements taking place in the primitive streak 

 of birds, especially at the level of the Hensen node, has been added by Spratt (1957). 

 -^ For the physico-chemical properties of this coat, see Bell (1958). 



^ H. Tiedemann (1957) has drawn arguments from the study of nucleotides in embryos 

 submitted to aerobiosis and anaerobiosis in favor of the role of ADP and ATP in gastrulation. 



Literature p. 483 



