462 WOODBURN: EMBRYOLOGY OF REBOULIA HEMISPHERICA 



reaching the egg the sperm nucleus is found to have the form and 

 structure of a nucleus in the resting stage (FiG. i). Quite similar 

 conditions have been reported by Black (i) and Garber (3) for 

 Riccia and by K. Meyer (4) for Corsinia. The form and structure 

 of the egg and sperm nuclei at this stage are to a certain extent 

 similar (FiG. i). The egg nucleus is larger and shows a coarser 

 and more open disposition of the chromatin. An unstained area 

 surrounds the nucleolus. A smaller nucleolus is present in the 

 sperm nucleus. The latter as a whole stains somewhat more 

 heavily than does the nucleus of the egg. 



The accompanying figures are drawn as nearly as possible in 

 that position which is occupied in nature. For instance, the 

 necks of the archegonia at this stage of development project obli- 

 quely downward or toward the substratum. Consequently it 

 will be seen that, in FIG. I, the sperm nucleus lies in contact with 

 that side of the egg nucleus which is toward the neck of the 

 archegonium. The membrane of the egg nucleus is somewhat 

 infolded along the surface of contact. 



Compared with corresponding stages in the life history of 

 gymnosperms and angiosperms we know very little concerning 

 the details of nuclear behavior in this and further stages of fer- 

 tilization. 



EARLY DIVISIONS OF THE EMBRYO 



The first division wall of the zygote is laid down transversely. 

 FIG. 2 represents the telophase of this first division, with the cell 

 plate in the process of formation. The spindle lies parallel with 

 the longitudinal axis of the venter of the archegonium; conse- 

 quently a transverse basal wall results (Fics. 2 and 3). In 

 FIG. 3 the nuclei are in the prophase of subsequent division-; 

 "a" represents the hypobasal cell or that one next to the base of 

 the archegonium, and b, the epibasal cell or that one next to 

 the neck of the archegonium. The second division is transverse 

 and occurs in the epibasal cell (compare FIGS. 3, 4, 5 and 6). 

 The epibasal cell (FiG. 3, "b"), with the chromosomes clearly dif- 

 ferentiated, is in a more advanced stage of prophase than the hypo- 

 basal cell. There is also evidence of centrosomes or centrosome- 

 like structures connected with the nucleus. FIG. 4 represents 

 the second division completed. Later stages (compare FIGS. 



