156 SCIENCE PROGRESS. 



ous other granules existing elsewhere in the egg. It has, 

 in fact, as far as its optical properties go, all the characters 

 of these " microsomes," being distinguished from them during 

 divisional metamorphosis solely by the position which it 

 occupies at the centre of affairs. 



The sphere in Ascaris was the first to be minutely 

 studied and described, and it has consequently served as a 

 sort of type for comparison with those we are becoming 

 familiar with on all sides. It is essentially composed of a 

 large mass of differentiated protoplasm outside the nucleus, 

 containing within itself a clear space and central body. 



The spheres are known in the literature which deals 

 with them under a great variety of names, " sphere-attrac- 

 tive " being practically synonymous (among others) with 

 "Sphere-directrice," "Aster," "Archoplasm," "Polar-body," 

 " Polar-corpuscle," and their German equivalents, " Pery- 

 plastes," with their " Daughter-peryplastes," " Microcen- 

 trums," " Spermocentres," "Ovicentres," " Nebenkerns," 

 " Astrocentres," "Vitelline nuclei" and " Dotterkerns ". 

 But although these titles have arisen in the description of 

 obviously homologous structures found in a great variety of 

 eggs and tissues, they by no means always refer to the 

 whole, or even to the same part of the three-zoned struc- 

 ture originally described by van Beneden. Thus the centro- 

 some is always the central staining granule, and is exactly 

 equivalent to "Polar-body," "Polar-corpuscle," "Daughter- 

 peryplaste," and " Microcentrum " (Heidenhain's (8) term 

 for multiple centrosomes in the aggregate). But how far 

 "Ovicentres," "Spermocentres," "Astrospheres" and "As- 

 trocentres," "Vitelline nuclei" and "Dotterkerns" represent 

 this and nothing more, it is difficult to say, while such 

 terms as " Sphere-directrice," " Aster," " Archoplasm," 

 " Peryplaste " and " Nebenkern " certainly stand for the 

 whole sphere, which, as in Ascaris, generally contains within 

 itself a smaller clear space and centrosome. The termino- 

 logy of Boveri (9) has the advantage of simplicity, since by 

 his use of " Archoplasm " for the outer, and " Centrosome" 

 for the inner constituent of the sphere, no mistake is likely 

 to be made. But as the archoplasm and the centrosome 



