78 THE REALISATION OF THE NUCLEAR FACTORS 



development was unmistakable. Other crosses, however, had 

 different results. In many cases, the hybrid was entirely similar 

 to the species to which the eggs belonged (so-called matrocline 

 hybrids), e.g. in the cross Sphaerechinus Q X Arhacia ^. In 

 other cases, the incompatibility of the paternal chromosomes 

 with the surrounding "foreign" cytoplasm manifested itself in 

 a somewhat dramatic way. In the cross Paracentrotus Q X 

 Arbacia r^ , the paternal chromosomes take a normal part in 

 the nuclear divisions during cleavage, but at the blastula stage 

 they are suddenly expelled into the cytoplasm where they dis- 

 integrate. From then on development is disturbed. We have 

 already seen that the cross Sphaerechinus X Psammechinus (or 

 Paracentrotus) gives normal hybrids of intermediate structure. 

 The reciprocal cross, however, Psammechinus (Paracentrotus) 

 X Sphaerechinus has an entirely different result. The paternal 

 chromosomes do unite with the egg nucleus, but in the first 

 or second division, 16 of the 20 Sphaerechinus chromosomes are 

 eliminated, and such development as takes place is matrocline. 



In the experiments here described, the activity of the paternal 

 chromosomes did not become visible until the pluteus stage. 

 Yet an investigation by Tennent (1922) has proved that even 

 before that stage they play a role in development. Tennent 

 crossed two sea urchins, belonging to the genera Cidaris and 

 Lytechinus. In Cidaris, the formation of the archenteron begins 

 20 hours after fertilisation, and the cells which will form the 

 skeleton disengage themselves from its wall a few hours after- 

 wards. In Lytechinus, on the other hand, the archenteron 

 invaginates after only 9 hours, and the skeleton-forming cells 

 arise from the vegetative wall of the blastula at an even earlier 

 time. In the hybrid Cidaris X Lytechinus, development first 

 takes a matrocline course, viz. until the end of the blastula 

 stage. However, the skeleton-forming cells originate from the 

 blastula wall, simultaneously with the beginning of the in- 

 vagination of the archenteron. On this point, therefore, the 

 influence of the paternal chromosomes expresses itself already. 



In amphibians, too, the result of crossing experiments 

 depends very much upon the nature of the species crossed. 

 Good development of the hybrids occurs, e.g., in the cross 



