CLEAVAGE AND DEVELOPMENTAL PATTERN 577 



ing factor of polarity is apparently not known. Sperm entrance in a region 

 about the basal pole suggests presence of a cytoplasmic pattern (cf . Dalcq, 

 1932c, 1935). Evidence of cytoplasmic dorsiventrality or bilaterality in 

 the unfertilized ascidian egg has been presented by van Beneden and Julin 

 (1884) and by Dalcq and Vandebroek (1937). 



Following sperm entrance, an extensive and remarkable series of cyto- 

 plasmic movements takes place in Styela (Conklin, 1905a). The yellow 

 ectoplasm first streams basipetally and aggregates in the region about the 

 sperm nucleus. Dalcq and Vandebroek (1937) find that local vital staining 

 indicates movement away from the apical pole of the egg cortex about 

 this pole in Ascidiella (see also Vandebroek, igs6b). The sperm nucleus 

 migrates to a position near the egg equator, but on one side of the egg 

 axis; and the yellow cytoplasm goes with it and finally forms a yellow 

 crescent transverse to the polar axis and symmetrical to the position of 

 the sperm nucleus (Fig. 182, A).''' This crescent, designated as "meso- 

 plasm" or "myoplasm," gives rise to muscle tissue. The indophenol 

 blue, benzidin, and leucomethylene blue tests for oxidases and perocidases 

 show no localization in the unfertilized egg, but, after fertilization, locah- 

 zation in the myoplasm (Ries, 1939; Reverberi e Pitotti, 1939). The path 

 of the sperm nucleus and formation of the crescent suggest a dorsiventral 

 pattern as a determining factor. Six organ-forming regions, indicated 

 by differences in the cytoplasm, are distinguished by Conklin.'^ 



Cleavage is, to a high degree, determinate. The plane of the equal first 

 cleavage coincides with the median plane and divides the yellow crescent 

 equally ; the second cleavage is vertical to it and also meridional ; the third 

 separates smaller apical from larger basal cells and is slightly oblique 

 bilaterally (Fig. 182, B). Later cleavages, so far as followed, are strictly 

 bilateral, and the cell lineage of organs and tissues is definite (Fig. 182, 

 C, D, E)."^ 



The study by Chabry of development of isolated blastomeres and blas- 

 tomere groups showed that cleavage and development were in all cases 

 partial and gave little or no evidence of reconstitution, except that ecto- 

 derm formed a complete surface layer. Driesch (1895) maintained, how- 



'7 The side of the egg on which the crescent Hes is regarded by Conkhn as posterior, but 

 comparison with amphibian development suggests that it is perhaps more nearly ventral or 

 posteroventral, since neural plate and notochord develop on the opposite side. 



•* Conklin, 1905(7, 191 1. See also Duesberg, 1913. 



'9 For earlier studies of ascidian cleavage see van Beneden et Julin, 1884; Chabry, 1887; 

 W. E. Castle, 1892. 



