396 



Transactions. 



into two more or less equal portions. I have never found any variation 

 from this. The surrounding prothallial tissue at this stage projects con- 

 siderably (fig. 34), so that it is possible sometimes to detect these young 

 stages in an external examination of the prothallus (see Plate LXIII, 

 fig. 1). Following this, there is formed in the inner or hypobasal cell an 

 inclined wall leading from the basal wall down towards the lower end of 

 the embryo and dividing the hypobasal portion into two somewhat unequal 

 quadrants. This stage is shown in longitudinal section in the series given 

 in figs. 35a to 35d and figs. 36a to 36d, and in transverse section in the 

 series figs. 37a to 37f. Next, a similarly inclined wall leads off from the 

 basal wall towards the upper end of the epibasal cell, though not into 



Figs. 30-34.- — Five young embryos in longitudinal section, showing first division -wall 

 only. The series 34a to 34c consists of consecutive sections, as in all 

 series of sections illustrated in this paper unless otherwise stated. X 100. 



the actual ' beak," the embryo thus attaining the complete quadrant 

 stage. This is shown in longitudinal section in the two series figs. 38a 

 and 38b, and figs. 39a to 39d, and in obliquely transverse section in the 

 series figs. 40a to 40g. This sequence in segmentation seems to be the 

 normal rule, so that before referring to the abnormal cases met with I 

 will describe the subsequent cell-divisions which lead up to the setting 

 apart of an apical cell in the epibasal region. 



I have not found a sufficient number of young embryos cut trans- 

 versely to determine whether or not there is normally a regular octant 

 formation, but judging from the embryo cut obliquely transverse and 

 illustrated in the series figs. 41a to 41g, and from others also, I should say 

 not. In this case in the lower portion of the hypobasal region the first 



