322 



INVERTEBKATA 



CHAP. 



questioned ; but when the structure of the embryos of Patella, 

 Trochus, and Chiton was elucidated, the real homology of velum 

 and prototroch became apparent. 



Another difference is seen in the development of the "cross." 

 This becomes far more conspicuous in the case of most Gastropods 

 than it does in Patella. The terminations of its four arms are formed 

 by the " tip cells " 2a n , 2b n , 2c n , and 2d n . The basal cells are of 

 course la 121 , lb 121 , lc 121 , and Id 121 , while tucked away between the 

 four basals and the apical cells la 111 , etc., are the so-called "peripheral 

 rosettes" la 112 , etc., which represent the Annelidan cross, but 



which do not divide more than 

 once in Mollusca and hence do 

 not attain any great development. 

 The basal cells of the Mollus- 

 can cross, on the contrary, divide 

 several times transversely, and 

 then the daughter cells in the a, 

 b, and- c quadrants become longi- 

 tudinally divided into two and 

 even into four rows of cells (Fig. 

 252). In the d quadrant they 

 remain undivided longitudinally 

 for a considerable time, but also 

 eventually divide, filling up the 

 gaps between the apical and the 

 prototrochal cells ; the latter 

 usually divide only once, forming 

 eight "turret cells," of which 

 only the anterior, as we have 

 seen, develop cilia. These points 

 can be well seen in the segmenting 

 egg of Crepidula (Fig. 253). 

 The most interesting thing that has been elucidated in the 

 development of these more modified Gastropods is the relation of the 

 organs of the veliger to certain groups of cells in the cell-lineage. 

 Thus, in Planorbis it is found that the cerebral ganglia arise by 

 internal proliferation from the lateral arms of the cross, except from 

 their tip cells and the cells immediately adjoining these ; from the 

 anterior arms, except from their tip and basal cells ; and from the two 

 hinder arms of the Annelidan cross. 



In Crepidula, Physa, and Fiona it is found that the mother cell 

 of the mesoderm, 4d, divides as usual into right and left cells, 4d r and 

 4d J . These then bud off two small anterior cells whose fate is to 

 become endodermic, while the mesodermic mother cells divide into 

 equal parts ; so that we have four large mesodermic mother cells, two 

 on each side. From each of the inner pair of mother cells a second 

 small cell is given off; and these, with the first two cells, form a 

 group of four small cells which lie close to the macromeres behind. 



FIG. 253. Apical region of an embryo of 

 Crepidula showing the Molluscan cross 

 in a late stage of development. (After 

 Conklin. ) 



The apical cells are unshaded, as are also the 

 primary trochoblast cells. The "peripheral 

 rosette " cells (Annelidan cross) are marked with 

 small circles. The cells of the Molluscan cross are 

 ruled with horizontal lines, except the derivatives 

 of the tip cells, which, since they belong to the 

 second quartette, are dotted. 



