41 6 HOLMES. [Vol. XVI. 



There are the same number of cells in each of the two rudi- 

 ments which, as far as the size and position of their component 

 cells is concerned, show a perfect bilateral symmetry. Yet the 

 derivation of the cells on the two sides does not exactly corre- 

 spond. The right rudiment contains cells from three quadrants, 

 while the left contains cells from but two. This is because 

 the intermediate cells from the d quadrant lie to the right of 

 the median axis of the ^%,Z- The posterior arm of the cross, 

 which is composed of cells of this quadrant, takes no part in 

 the formation of these structures. The two products of the 

 cleavage of the intermediate cell and the apical i^ '■'■', which 

 enters the apical plate, are the only cells of the first quartette 

 in the d quadrant which do not go to form the head vesicle. 



The cells forming the cerebral rudiments multiply with great 

 rapidity. The areas become thickened and a proliferation of 

 cells occurs which gives rise to the cerebral ganglia. The 

 early appearance of these rudiments was observed by Rabl, 

 who designated them a bilobed apical plate (Scheitelplatte). 

 The further history of the fundaments of the cerebral ganglia 

 may be followed in Rabl's paper. The areas above described 

 are not exclusively employed in the formation of the cerebral 

 ganglia and eyes. The cells become so numerous before differ- 

 entiation begins that it is impossible to trace the cell origin of 

 the different structures arising from them. As the eyes arise 

 at the outer sides of the cerebral ganglia, it is quite certain 

 that they are formed from cells derived from the lateral arms 

 of the cross. Whether this can be said also of the tentacles is 

 uncertain. 



The Apical Plate. 



The term "apical plate" has been applied by Conklin to a 

 median belt of large, clear cells in Crepidula extending from 

 the apical sense organ to the prototroch. It is composed of 

 seven cells which become covered by fine cilia and remain for 

 a long time undivided, while the neighboring cells rapidly mul- 

 tiply. In Planorbis there is a median belt of large cells extend- 

 ing from the head vesicle to the prototroch, and which, from its 

 similarity to the apical plate in Crepidula, I have designated by 



