54 Bashford Dean Memorial Volume 



shown in Figures 14 and 15 of plate II resemble each other in the general character of 

 their cleavage patterns, and in the fact that each contains nine blastomeres. 



Text-figure 5 shows an eight-celled stage reproduced from Dean's 1899 memoir. 

 This figure was drawn from reconstructions of serial sections, and concerning it Dean 

 says— ^^In three of the blastomeres of a preceding stage vertical cleavage has occurred, 

 while in the fourth (that at the right of the figure) a nearly horizontal cleavage is demon- 

 strated by sections." This blastodisc illustrates the general tendency for the third cleav- 

 age furrows to form vertically. 



DEAN-S DRAWINGS (IN POLAR VIEW) OF WAX-PLATE RECONSTRUCTIONS 



FROM SERIAL SECTIONS OF BLASTODERMS OF BDELLOSTOMA STOUTI, 



REPRODUCED FROM HIS 1899 MEMOIR 



Text-figure 4. Second cleavage stage, slightly irregular. 



Text-figure 5. Third cleavage stage, somewhat irregular. Of the third cleavage furrows, two do not 



show in the figure. 

 Text-figure 6. Early fourth cleavage, rather irregular. 

 Text-figure 7. Late cleavage, micromeres somewhat unequal in siz;e. 



The germinal hillock shown in lateral view in Figure 16, plate II, has evidently reached 

 a late third cleavage stage. So far as can be determined from this lateral view the cleavage 

 pattern is somewhat irregular. In both Figures 13 and 16 of this plate it is plainly noticeable 

 that the form of the germinal hillock is now that of a very much flattened dome. Study of 

 all the figures drawn in lateral view reveals a process of reduction in the height of the hil- 

 lock and a broadening of its base which presumably continues throughout the later seg- 

 mentation stages. This elevation of the germinal region in the form of a conspicuous hil- 

 lock which only gradually flattens out, is one of the most unique features in the early 

 development of Bdellostoma. Neither of us has ever encountered such a structure in any 



other animal egg. .11 



The cleavage pattern portrayed in Figure 17, plate II, is more irregular than any yet 

 studied-so much so that the cleavage furrows cannot be identified. There are four or 

 possibly five micromeres completely outlined in the blastodisc. Two of these are exceed- 

 ingly small, the others are about normal sue. The side of the egg toward the observer 

 appears to be more developed than the opposite side. 



