110 



INVERTEBRATA 



CHAP. 



in eggs with spiral cleavages ; it has been observed both in 

 Annelida and Mollusca. 



We must now return to a more detailed study of the three 

 ectodermal quartettes. Turning our attention to the first quartette, 

 we left it at the stage where it consisted of sixteen cells, viz. la 11 -!^ 11 , 

 la 12 - Id 12 , la 21 - Id* 1 , and la 22 - Id 22 ; la 11 is rather larger than 

 la 12 , and la 21 is larger than la 22 , la ll -ld n bud off four small cells 

 which occupy the uppermost pole of the egg, these are the so-called 

 apical cells, la ni -le m ; they are little more than nuclei, the 

 cytoplasm being reduced to a thin plate in which cell limits are 

 not discernible (ap, Fig. 85). 



Now this type of division by which small sisters are separated 

 at the surface whilst larger sisters remain more deeply situated, is 



repeated afterwards by the 

 larger daughter cells which 

 have resulted from the division 

 just described. These cells, 

 la 112 -!^ 112 , divide each into an 

 upper very small cell, la 1121 - 

 lo! 1121 , and a lower larger cell, 

 Ia ll22 -ld 1122 . For the third 

 time a similar division takes 

 place. The lower cells resulting 

 from this last division, la 1122 - 

 ld 1122 , give rise to four very 

 small cells situated externally, 

 and to four much larger cells 

 situated internally. 



The larger daughter cells 



arising from the last division 



FIG. 85. Developing egg of Planocera inquilina 

 in a late stage of segmentation, viewed from 

 animal pole. (After Surface, slightly altered. ) 



are denominated 



ap, thin apical plate without distinction of cell , 



outlines formed by the cells o"i, 6"i, d", and <pn. <JnC6 again they Undergo a 



The dotted lines indicate concentric circles of small similar division each giving 



cells near the animal pole of the egg A third circle j t } internal Cell and 

 is constituted by the cells composing the apical plate. 



a small external one.' 



Thus we finally get four concentric circles of small cells at the 

 upper pole of the egg, whilst the four larger internal cells resulting 

 from the last division form the rudiment of the cerebral ganglion 

 (g, Fig. 84). The cells, Ia 121 -ld 121 , divide each into two daughter cells 

 of equal size, but the cells, l 21 -ld 21 and la 22 -!^ 22 , divide each into a 

 large internal and a small external cell. 



Turning now to the second quartette, each member of course 

 divided at the time that the third quartette was given off, for until the 

 32 -cell stage all the cells of the egg divide together. Thus we 

 have eight cells, 2a l -2d l and 2a?-2d'*; if we take the divisions in 

 quadrant as an example of the whole we find that 2a l , divides into a 

 smaller external cell, 2a 12 , and a larger deeper cell, 2a n , whilst 2a 2 

 divides into two equal cells one above the other. The lower of these 



