194 CYTOLOGY 



CHAP. 



substance of the body, reacts upon the cytoplasm on which, in the last 

 instance, the forms and functions of the cells, and therefore of the 

 organism, directly depend. , 



This view is again largely founded upon the analogy of the Infusoria, 

 where the nucleus is divided into two portions, the micronucleus, which 

 alone takes part in conjugation, and the macronucleus, which is supposed 

 to be concerned with the physiological activities of the cell, and which 

 disappears before conjugation, a new one being derived from the micro- 

 nucleus after syngamy. The ordinary Metazoan nucleus contains both 

 kinds of nuclear substances, the trophochromatin being separated from 

 it from time to time as required in the form of chromidia, which therefore 

 correspond to the Infusorian macronucleus. The only nuclei in the 

 Metazoa which exactly correspond with the Infusorian micronucleus, 

 consisting entirely of idiochromatin, are the gamete nuclei, from which 

 all the trophochromatin is supposed to have been eliminated during the 

 growth period of the oocyte or spermatocyte (Goldschmidt, 1905, 1910). 



While probably no useful purpose is to be served by labouring the 

 distinction between the two kinds of nuclear substances (since in any 

 case the one is directly derived from the other), if it were established 

 that the nucleus exerts its morphogenetic action through the agency 

 of chromatin extruded naked into the cytoplasm, a step, though a small 

 one, would undoubtedly have been taken towards the understanding 

 of this dark problem. Practically our only direct evidence in favour 

 of this view is to be found in a series of papers by Schaxel. 



His results in a large number of forms are remarkably uniform, and 

 may be illustrated by the development of the Polychaete Aricia foetida 

 (1912) and Strongylocentrotus (1911 a). 



There is no emission of chromidia during the process of cleavage 

 (cell multiplication), but before cell differentiation begins, e.g. before 

 the endoderm cells take on the specific character of the cells of the 

 alimentary canal, or mesoderm cells develop into muscle cells, an emission 

 of chromatin takes place. 



To take a specific example : the conversion of an undifferentiated 

 mesenchyme cell of Strongylocentrotus into a skeletal cell is preceded, 

 according to Schaxel, by an emission of chromidia from the nucleus 

 into the cytoplasm. These chromidia congregate into a mass, in the 

 middle of which a globule of the skeletal secretion soon appears. This 

 increases in size at the expense of the chromidia — that is to say, the 

 chromidia are destroyed or used up by their own formative action, 

 though it is not suggested that they are actually transformed into the 

 secretion. As the Echinoderm skeleton is extracellular, this secretion 

 has to be extruded from the cell to take part in the formation of the 

 skeletal spicule. At present the work of Schaxel stands in need of con- 



