IV 



COELENTEKATA 



91 



mes 



blastomere takes nearly an hour to accomplish. , During the process 

 of the division of the eight blasto- 

 meres into two tiers of cells, the 

 cytoplasm flows from the daughter 

 cell, which is originally the larger, 

 upwards into the smaller cell ; so, 

 by the time the division is accom- 

 plished the original proportions of 

 the two cells have become re versed, so 

 that what was originally macromere 

 is now micromere and vice versa. 



At the next cleavage eight more 

 micromeres are budded off; this is 

 the division of the macromeres 

 of the 16-cell stage ; and the first- 

 formed micromeres divide, and thus 

 the cleavage is complete, every cell 

 in the egg having divided, and a 

 total of thirty-two cells has been 

 reached. Each macromere with the 

 micromeres to which it gives rise, 

 may be termed an octant of the 

 egg. In Fig. 69 the egg of another 

 Ctenophore is shown in this stage 

 of development, seen from the side. 

 All Ctenophora, the development 

 of which has been examined, seem 

 to agree in the way in which the 

 cleavage of the egg is carried out, and 

 this figure may therefore be taken as 

 representing what goes on in Beroe. 



In the outer octants the division 

 of the first-formed micromeres is 

 unequal, the smaller daughter cell 

 being the smallest of all three 

 sets of micromeres, and the larger 

 daughter the largest of them; the 

 new micromeres which have just 

 been budded from the macromeres 

 being intermediate in size. In the 

 middle octants the first -formed 

 micromeres divide equally. 



In the next period of cleavage 

 the micromeres alone divide, the 

 macromeres remaining quiescent. 

 The smallest micromere divides into two equal parts. Its sister cell 

 separates off towards the upper pole as a smallest tertiary micromere, 

 and, as all the daughters of the first-formed micromeres in the middle 



stom 



FIG. 72. Illustrating the origin and fate 

 of the so-called mesoderm in a Cteno- 

 phore embryo (Collianira bialata). 

 (After Metschnikoff. ) 



A, view from oral pole at the time when the 

 " mesoderm " is being budded off. B, optical 

 longitudinal section of a slightly later stage, 

 showing rotation of the macromeres and the 

 consequent invagination of the " mesodenn." 

 C, optical section of a still later stage, showing 

 accumulation of mesoderm at upper pole and 

 formation of stomodaeum. Letters as before. 

 mes, mesoderm. 



