i6o CYTOLOGY chap. 



maternal in character, but with apparently the full complement of 

 chromosomes), it may be accepted as reasonably satisfactory. 



Elimination of chromosonies in cleavage of other Echinoderm crosses 

 has also been described by Tennant (1912) and by Doncaster and Gray 

 (1913). The results of Baltzer's crosses can be tabulated as follows : 



(i) Development is normal. There is no elimination of chromosomes 

 or chromatin. Plutei are intermediate between the two parents, 

 [Echinus $ x Strongylocentrotus ^ , Strongylocentrotus $ x Echinus ^ , 

 Sphaerechinus $ x Echinus ^ , Sphaer echinus $ x Strongylocentrotus ^ ) . 



(2) Development is pathological. Most of the parental chromosomes 

 are eliminated in the first two cleavages. Plutei are of maternal type 

 [Echinus $ x Sphaerechinus ^ , Strongylocentrotus 9 x Sphaerechinus ^ , 

 Arbacia ? x Echinus 6 , Arhacia ? x Strongylocentrotus ^). 



(3) Development is pathological. The chromatin is ehminated in 

 the blastula stage. The plutei are of maternal type [Echinus $ x 

 Arhacia ^ , Strongylocentrotus 9 x Arhacia ^ ). 



(4) Development is pathological. No chromatin is ehminated. The 

 plutei are maternal, either predominantly [Sphaerechinus^ x Arhacia ^) 

 or completely [Echinus $ x Antedon ^ , Strongylocentrotus 9 x Antedon ^ ). 



It will be noticed that the first three of the above results accord well 

 with the view that the idioplasm is exclusively contained in the nucleus, 

 classes 2 and 3, if finally established, even constituting strong evidence 

 in favour of that hypothesis. As regards the 4th class, the following 

 considerations suggest themselves. 



[a) All these three crosses, it will be noticed, are between distantly 

 related genera — in the case of the two last indeed, between different 

 classes. Now, granting that the idioplasm is contained in the nucleus, 

 it can only exert its morphogenetic function through, and on, the 

 cytoplasm. When idioplasm finds itself in such a strange environment 

 as cytoplasm belonging to a distantly related genus, or even class, it is 

 not unlikely that it may be unable to exert any of its normal functions, 

 though able to nourish and maintain itself, as is shown by the fact that 

 the parental chromosomes multiply in mitosis with the maternal. 



Extreme examples of the behaviour of chromatin in foreign cytoplasm 

 are furnished by the crosses made by Kupelweiser (1909). In these 

 the male chromosomes were unable even to maintain themselves and 

 multiply. He succeeded in fertihzing eggs of Echinus with sperm of 

 Molluscs and Annehds (Fig. 73). The sperm nucleus did not fuse with 

 the egg nucleus, but remained practically unchanged in the egg cytoplasm, 

 obviously inert. The egg indeed developed, but the role of the sperm 

 in this case was plainly merely to supply the stimulus to development. 

 These cross fertihzations may indeed be considered as another of the 

 methods of bringing about artificial parthenogenesis. Under these 



