IV FINAL CONSIDERATIONS 481 



the phosphatases. Their association clearly points to the general involvement of 

 high-energy phosphorus bonds ; adenosine di- and triphosphate and similar com- 

 pounds have remained here in the background because of our inability to detect 

 and localize them in the cells or blastomeres^ However, a recent theoretical paper 

 devoted to movements in embryonic cells (Mulnard, 1956) has shown how many 

 reasons we have for suspecting the role of the high-energy phosphorus compounds 

 and the corresponding enzymes in all the processes with which we have been 

 concerned. 



The finaUty of development is obviously the construction of associated organs 

 and tissues with their appropriate cytodifTerentiations^. The individualization of 

 organs is clearly the result of morphochoresis, while cytodifferentiations proceed 

 from more varied causes: (j) the intrinsic constitution of some cells, e.g. in deriv- 

 atives of chordo-mesoblast or endoblast; a concept perhaps open to revision since 

 it is not demonstrated yet that cultured cells of these embryonic anlagen will 

 produce a completely homogeneous lineage; (2) inductive processes which may 

 (a) primarily influence the form of the cells, as in neuroblasts; (b) evoke, probably 

 with the collaboration of the mitochondria, definite thread-like structures due to 

 chains of macromolecules for example neurofibrils at a later stage of the neuroblasts, 

 myofibrils (in the experimentally induced myoblasts), and the fibrils of ciHa, such 

 as those in the epidermis of tadpoles and in the sensory patches of the inner ear; 

 {c) provoke the synthesis of specialized proteins such as lens protein, myosin and 

 actomyosin, etc.; (j) possible activity of the centriolar apparatus, which may 

 well be more extensive than the production of ciHa or flagellae, and whose asso- 

 ciations with the Golgi apparatus may be more frequent than has been observed 

 (as suggested by the origin of the acrosomal substance) ; {4) hormonal influences 

 clearly apparent in the reproductive organs and directly demonstrated in vitro for 

 vitamin A (Fell and Mellanby, 1953; Fell, 1954; Weiss and James, 1955). Thus, 

 it may be said that differentiation is mostly provoked, and even that its occurrence 

 as an autonomous or "spontaneous" process could be questioned. 



The above schematic enumeration is merely a glance upon the vast and 

 promising problem of differentiation. For additional recent information, the con- 

 structive essay of Weiss (1953) and the proceedings of the Providence conference 

 (1956) should be consulted (Rudnick, 1958). 



It is important to emphasize that, in an impressive number of cases, cytodifferen- 

 tiation can now be produced by introducing definite substances of the organizin 

 or vitamin type into the cell milieu. It should be stressed also that some puzzling 

 processes occurring in adult organisms will probably benefit from studies conducted 

 with the basic mechanisms of development in mind. One good example is osteo- 



^ I have recently been able to detect cytochemically adenosine-triphosphatase in mouse, rat 

 and mole eggs. The activity of this powerful enzyme, probably located in the mitochondria, 

 is far from being constant throughout early development. This new approach seems quite 

 promising. 



^ For the elaborate character of the properties acquired by differentiation, observations 

 by Weiss and Moscona (1958) are especially striking. If the mesenchymes derived from the 

 sclerotic and from the limb bud are submitted to dissociation and reaggregation, they 

 preserve their respectively bi- and tridimensional arrangement. 



Literature p. 483 



