from the di\asion of one cell lie at an equal distance 

 from the pole. In this case the letters r for right 

 and 1 for left are used. The practice is, however, 

 not generally followed. 



Descriptions of various types of cleavage can be 

 found in volume 1 of MacBride (1914). The equal 

 and unequal cleavages in the spu-ally cleaving eggs 

 of annelids and molluscs are discussed by Costello 

 (1955) in chapter 2 of Willier, Weiss, and Ham- 

 burger (1955). 



During division the micromeres of a quartet, 

 viewed from the animal pole, become slightly dis- 

 placed because the spindles of the dividing cells 

 (not shown in fig. 324 or 325) occupy an oblique 

 position with respect to the egg's axis. At the 

 following di\T[sions the plane of separation of 

 daughter cells is oriented approximately at a right 

 angle to the preceding divisions. The pattern of 

 such cleavage is called spiral. It gives rise to an 

 irregular morula (sterroblastula according to Kor- 

 schelt and Heider, 1895, from the Greek "sterros" 

 meaning firm) found in annelids (Nereis), in some 

 bivalves (Ostreidae, Teredo), and in gastropods 

 (Crepidula, Fulgur, Nassa), and others. In all 

 cases the sterroblastula arises from an unequal 

 cleavage dm-ing which the micromeres overlie the 

 macromeres, and at each division are slightly dis- 

 placed to the right (dexiotropic cleavage) or to the 

 left (laeotropic cleavage). Sometimes, as in the 

 case of Dreissensia, the second dexiotropic cleavage 

 is followed by a third dexiotropic cleavage after 

 which the normal alternating coiKse is established 

 (Meisenheimer, 1901a). In the case of oyster 

 eggs, as shown by Fujita (1929) for C. gigas, the 

 cleavage is laeotropic. 



The multicellular stages of a C. virginica egg are 

 reached in the course of the sixth and ensuing 

 cleavages (figs. 326 and 327) during which the 

 micromeres divide much more rapidly than the 

 macromeres, become progressively smaller and 

 overgi-ow the vegetal pole. Approximately at this 

 stage the sterroblastula of an oyster Is formed. 



Gastrulation begins with epibolic extension of 

 the micromeres. At 22° to 24° C. the stage 

 shown in fig. 328 is reached within 4 to 6 hours. 



The cell lineage of C. virginica has not been 

 studied; the stages of development of eggs of the 

 species shown in figm'es 320 to 328 are similar 

 to those previously described by Brooks (1898); 

 Horst (1882) for O. ediilis; Seno (1929) for 0. 

 denselamellosa; Hori (1933) for 0. lurida; Yasugi 

 (1938) for 0. spinosa, and C. gigas. Yasugi 



Microns 



Figure 326. — Formation of sterroblastula in C. virginica 

 egg; of the four macromeres only two are visible from 

 the side. Micromeres begin to overgrow the vegetal 

 pole. Drawn from photomicrograph of live egg. 







M icrons 



20 



Figure 327. — Advanced sterroblastula stage of C. vir- 

 ginica. One of the macromeres has divided into two 

 daughter cells of equal size. Drawn from photomicro- 

 graph of live egg. 



EGG; SPERM, FERTILIZATION, AND CLEAVAGE 



347 



