Microns 



Figure 323. — Third division of fertilized egg of C. vir- 

 ginica; side view. Separation of mieromeres (on top) 

 from macromeres (bottom). Only one macroraere is 

 seen in the plane of view. Live egg. 







Microns 



20 



Figure 324. — Fourth cleavage of egg of C. virginica and 

 the formation of 8 mieromeres (2d quartet). Side view. 

 Drawn from photomicrograph of live cell. 



their ultimate fates as parts of the larva or adult, 

 was first described by Wliitiuan (1878) for tlie 

 egg of C'kpsine, and followed by Wilson (1892) for 

 the egg of Nereis. The works of Lillie (1895) 

 on the development of Unionidae, C'onklin (1897, 

 1908) on Crepidula and Fulgur, Meisenheimer 



Microns 



20 



Figure 325. — Fifth cleavage of egg of C. nirginica and 

 the formation of the third quartet of mieromeres (32- 

 cell stage). Side view. Drawn from photomicrograph 

 of live cell. 



(1901a, 1901b) on Dreissensia and Cydas, and 

 Wilson (1904a, 1904b) on Dentalium and Patella 

 constitute major contributions to the embryology 

 of mollusks. 



The nomenclature of the cleavage blastomeres, 

 as developed by Wilson (1892), was progressively 

 modified by Conklin (1897), Mead (1897), and 

 Child (1900); the present system is based largely 

 on the work of Robert (1902) on the development 

 of the Trochti^ egg. The system is a combination 

 of letters and numbers by which the blastomeres 

 are identified. 



The fu'st four cells or macromeres are designated 

 as A, B, C, and D; in the majority of cases studied 

 D is the largest of the four and is situated at the 

 side which will develop into the posterior portion 

 of the embryo. When the first four blastomeres 

 divide, their daughter cells are denominated la and 

 lA, lb and IB, Ic and IC, and Id and ID, the 

 small letters in each case referring to the micromere 

 and the capital letter to the macromere. 



In successive dixisions lA divides into 2a and 

 2A, IB into 2b and 2B, and so on. When the 

 mieromeres divide, la is divided into la' and la^, 

 tlie superscript 1 denoting the daughter cell which 

 is nearest to the animal pole and superscript 2 the 

 one nearer to the vegetal pole. The nomenclature 

 is capable of indefinite expansion, but certain con- 

 fusion arises when the two daughter cells resulting 



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