MARATTIALES 



281 



Upper third is colourless and nearly hyaline. This is the 

 "receptive spot," and it is here that the spermatozoid enters. 

 The nucleus is of moderate size, and not rich in chromatin; a 

 small but distinct nucleolus is present. The spermatozoid 

 retains its original form after it first enters the egg, and until it 

 comes in contact with the membrane of the egg nucleus. It 

 afterwards contracts and assumes much the appearance of the 

 nucleus of the sperm cell previous to the differentiation of the 

 spermatozoid. The two nuclei then gradually fuse, but all the 

 different stages could not be traced. Before the first division 



A..<^r7T7>x ^-^^ . B. 



Fig. i54.->-MafaHio Douglasii. Embryogeny. A, Longitudinal; B, transverse sections 

 of embryos, X215; C, vertical section of an older embryo, showing its position in 

 the prothallium, - X72; st, the stem; pr, prothallium; D, upper part of the same 

 embryo, X215. 



takes place, however, but one nucleus can be seen, and this 

 much resembles the nucleus of the unfertilised egg. It is prob- 

 able that the nucleus of the spermatozoid really penetrates the 

 cavity of the egg-nucleus as has been shown to be the case in 

 Onoclea. ( See Shaw ( i ) ) • 



The Embryo — (Farmer (s) ; Jonkman (s)) 

 After fertilisation the egg enlarges to several times its 

 original size before dividing. The first (basal) wall is trans- 



