300 BOTANICAL GAZETTE [may 



Sporophytic outgrowths begin very early from cells in the region 

 where later the cushion arises, so that the development of the sporo- 

 phytic outgrowth and the gradual completion of the cushion proceed 

 side by side for a while. When the prothallium has assumed the 

 characteristic heart shape, with a cushion near the sinus and an 

 extensive lateral growth of prothallial tissue on either side, the sporo- 

 phytic outgrowth is usually in a well-advanced stage. The main 

 features of the formation of this outgrowth and of the gradual com- 

 pletion of the cushion are as follows: 



Previous to the formation of the cushion, mitoses take place in 

 rapid succession in the vicinity of the sinus, partition walls always 

 being laid down perpendicular to the surface of the prothallium and 

 parallel to one another, so that the cells formed are very narrow- 

 Some of these mitoses in different stages are represented in figs. q-ii. 

 Mitosis continues and cell plates are laid down parallel or oblique 

 to the surface of the prothallium, the ultimate result being the initia- 

 tion of a thick cushion region (figs. 12, ij). 



One of the superficial cells in the cushion region begins at once to 

 increase considerably in size, the increase being accompanied by an 

 excessive growth of its nucleus. The nucleus in the resting condi- 

 tion contains a reticulum of ragged clumps and slender threads of 

 chromatin, from which the spirem of the prophase is established 

 (figs. 14, 14a). Two or more nucleoli are always present. Succes- 

 sive stages of the mitosis following the early prophase were examined 

 (figs. 13-17), which were exactly similar to those of typical mitosis 

 in the vegetative cells of normal prothallia. In the telophase of this 

 mitosis, when the two groups of daughter chromosomes have reached 

 the poles, a little irregularity in the form of the chromosomes is 

 observed, but the number of chromosomes, before they had become 

 aggregated into a mass, was always 64 or 66 (figs. 18a, b). 



Consequently, it is perfectly clear that, so far as the chromatin 

 is concerned, no change has occurred in the nucleus of these pro- 

 thallia up to the formation of the superficial cell. In the late telo- 

 phase a cell plate is laid down perpendicular to the surface of the 

 prothallium, so that there are formed two superficial daughter cells 

 arranged side by side (fig. ig) . 



A number of mitoses follow in the same way, and thus there is 



