APOGAMOUS FERNS. THE GENERAL PHENOMENON 



that characteristic of the rest of the sporophyte, but the haploid state is not thereby 

 attained. 



(3) The third type of sporangium is a variant on that just described and is in a sense 

 an imperfect version of it. The cytoplasmic activities which normally result in cell 

 cleavage may not be entirely suppressed but may be present in unco-ordinated forms 

 which affect in a very striking way the shapes and behaviour of the resulting mother 

 cells. Sporangia of this kind are those which were interpreted as stages of nuclear migra- 

 tion and fusion when first seen. A superficial resemblance to such a process they un- 

 doubtedly possess. The nuclei, after loss of the spindle, may become irregularly lobed 

 (Fig. i6ga, b and d), cell walls partially crossing the cell may be laid down, often in 

 relation to such lobes (cf. Fig. 169 a, b), and sometimes complete cleavage into two 

 unequal portions containing different-sized pieces of the restitution nucleus may result 



3^* m 



\ 



yr ■**# 







Fig. 168. Meiosis in Cyrtomium falcatum (L.f.) Presl from sections, x 1000. 

 a. Diakinesis. b. The first metaphase. 



(Fig. 169 c,/, g). That the cleavage, in the last event, involves a passive amitotic con- 

 striction of a restitution nucleus and not a mere inequality of anaphase distribution of 

 normally separating half-chromosomes is proved by the complete regularity of chromo- 

 some pairing no matter how large or small a piece of nucleus may have been con- 

 stricted off; this could not be attained by any process of random distribution of 

 non-homologous half-chromosomes but must denote the random separation of groups 

 of split chromosomes with their halves still in close contact. As may be seen from 

 several of the figures quoted, which refer to a number of different species, not all 

 the mother cells in a sporangium may be affected in this way, but only one or a few. 

 Since the distribution of chromosomes to the constricted portions is certainly at 

 random the nuclei so formed can hardly fail to be genetically unbalanced, and 

 abortion of the resulting spores is therefore virtually certain. 



(4) The fourth type of sporangium was not observed by Dopp, but I have come across 

 it from time to time in almost every species. Occasionally the abnormality affecting the 

 premeiotic division in the eight-celled sporangia may occur twice running and affect the 

 four-celled stage also. In such cases only four giant mother cells are to be found at 



166 



