450 C. C. MACKLIN. 



Furthermore, after the critical point is passed the division occurs 

 very rapidly. 



The study of fixed and stained specimens served to throw con- 

 siderable light upon the process of direct division, for, by searching 

 the field, transitional forms were encountered, suggesting stages 

 in the history of the living nucleus just described. In the ter- 

 minal stage of direct nuclear division, such as that shown in Fig- 

 24, mitochondria were found characteristically lying across the 

 slender strand of nuclear membrane which was all that remained 

 of the connection between the two portions of the nucleus, and 

 the centrosphere (which is undivided in these transitional forms, 

 and also in the end product of nuclear amitosis, the binucleate 

 cells) is situated in the cleft dividing these parts. 



This process involves a more or less equal mass division of the 

 nucleus without chromosome formation. It results in the pro- 

 duction of a binucleate cell, and, if the process is repeated, of a 

 trinucleate or multinucleate cell. Direct nuclear division was 

 not observed beyond the bipartite stage, but it seems rational to 

 suppose that (excluding the foreign body giant cells of Lambert 

 (1912 a and b) the giant cells of tissue cultures are formed from 

 the successive direct splitting of the nuclear fragments which 

 become larger by normal growth. With this latter there is 

 apparently associated an increase in the cytoplasm also. 



It was not practicable to make a complete study of amitosis 

 upon the same cell, and the latter stages of this process, succeeding 

 direct division of the nucleus, were studied by selecting living 

 binucleate cells and making prolonged observations upon them. 

 A typical case, from a 24-hour culture of 8-day chick heart, is 

 given as follows: 



11.50 A.M. The cell, which was of the characteristic connective tissue type, 

 showed two separate nuclear sacs, whose adjacent surfaces were in close contact. 

 One sac contained three nucleolar fragments, the other one. Fat granules were 

 fairly abundant, and were principally congregated at the nuclear poles. Mito- 

 chondria, long and threadlike, stretched between these granules and, where the 

 latter were abundant, the strands of mitochondria tended to arrange them in rows. 

 Mitochondria also radiated from the single centrosphere, situated opposite the 

 area of contact of the two nuclear sacs. The triangularly shaped cell body was 

 connected with adjacent cells by three main processes. 



12. 20 P.M. A narrow interval can be seen between the nuclear parts, showing 

 that the latter have moved apart and are quite separate. Their position also has 



