530 PROCEEDINGS OF THE ACADEMY OF [DeC, 



be dealt with later. From the fact that this cell gives rise to both 

 endoderm and mesoderm we may call it the " Mesentoblast " (Conklin, 

 97). 



History of the First Quartet — Apical Cells. 



After the first two macromeres, 4B and 4D, are formed and before 

 the other two cells, 3/1 and 3C, have divided, spindles appear in the 

 four stem cells of the first quartet, la^-^-ld^-^. The division of these 

 cells takes place in a very marked laeotropic direction. The results of 

 this division are four very small cells, la^-^-^-ld^*-^, lying at the proximal 

 end of the spindle or just over the animal pole, and four much larger 

 distal cells, W-'-'-ld'-'\ As fig. 18, PI. XXXVII, shows, the Inotropic 

 direction of the spindle is so marked that the cell la^-^-^ comes to lie in 

 front of Id^-^'^ and 1¥-^-^ in front of la*-^-^ and so on, so that there is a 

 rotation of 45 degrees. 



These small cells at the animal pole have been observed by Selenka 

 and Lang in polyclads, and designated by Selenka as the apical or crown 

 cells (Scheitelzellen). Selenka described these cells as arising from the 

 stem cells of the first quartet, i.e., la*-^-l<i^-^, as I have done. Lang, 

 on the other hand, says that they arise from the cells which alternate 

 with these stem cells, i.e., ae^, he^, ce^ and de^ of his system, or la*-^, 

 16^-2, Ic^-^ and Id^-^ of our system. He states that these cells W-^-ld^-^ 

 send in processes between the stem cells, and from the ends of these 

 processes the small apical cells arise. Lang describes this process in 

 considerable detail, and while it is possible he is right for Discoccelis 

 tigrina, I think it is very unlikely. 



Lang's observations were all made on the living eggs and not checked 

 with stained material. I first observed these divisions in the living 

 egg of Planocera and came to the same conclusion as Lang, viz., that 

 the apical cells came from the cells la^-^-ld^'^. It was only later when 

 I studied the preserved material of this stage that it became unmis- 

 takably evident that it was the stem cells la^-^-ld^-^ that were dividing 

 (fig. 18). The appearance in the living egg is very deceptive owing 

 to the great angle through which the spindles turn and to the small size 

 of the resulting apical cells. 



These apical cells consist of a well-formed, normal-sized nucleus and 

 a very small amount' of cytoplasmic material. In the stained eggs it 

 is only seldom that one can see anything at all of the cell boundaries. 

 Consequently in most of the drawings only the nuclei of these cells are 

 represented. 

 I, In their method and time of origin and in their relative size these 



