PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 145 



apical ovarian regions consisting of the younger follicles and from the 

 outer ovarian surface inward. In older single follicles in which the grow- 

 ing oocyte is distinguishable from nurse cells in living material, the nurse 

 cells reduce before the oocyte, and reduction in the oocyte and in the 

 follicle wall progresses basipetally. In the full-grown egg cell a basipetal 

 reduction gradient usually appears, and some evidence of a ventrodorsal 

 gradient has been obtained; but this gradient, if present, is slight. Since 

 the data show a uniform and constant relation between morphological 

 and physiological pattern of ovary, folhcle, and egg, they suggest that 

 the ovarian differential determines which cell in the follicle becomes 

 egg and the axiate pattern of that egg. 



EARLY CHORDATE DEVELOPMENT 

 ASCIDIANS 



A polar-dorsiventral pattern is directly visible in some ascidians in 

 differences in color and appearance of certain cytoplasmic regions, but 

 this pattern appears only after fertilization. Moreover, the visible pat- 

 tern is of no significance for ascidian development, for in other species no 

 such pattern is visible, but development follows the same course, and a 

 polar-dorsiventral pattern is inferred.'" But it is perhaps worth repeating 

 that, whatever the egg pattern is, it is not necessary for ascidian develop- 

 ment. Ascidians can develop from pieces of the body, from stolon tips 

 and isolated pieces of stolon, from buds, and from aggregations of cells. 

 Evidently, the regional cytoplasmic pattern of the ascidian egg is a prod- 

 uct or expression of a more fundamental pattern of some sort; that is, 

 more or less regional differentiation occurs in this egg before cleavage be- 

 gins. Apparently, the only direct evidence of gradient pattern in ascidian 

 development is the differential reduction of potassium permanganate in 

 Corella willmeriana (Child, 1927(f). In early cleavage the reduction gra- 

 dient is distinctly basipetal (Fig. 50, A). In the gastrula a region corre- 

 sponding to the dorsal region of the amphibian embryo (see Fig. 156) 

 in its relation to the blastopore, though not in relation to the primary 

 polarity of the egg, reduces most rapidly, with lateral margins of the 

 blastopore following (Fig. 50, B, C). Even before outgrowth of the tail, 

 increase in rate of reduction appears in the region of the prospective tail 

 (Fig. 50, D), and the longitudinal gradient becomes two-armed or U- 

 shaped; that is, a new gradient, opposed in direction to the primary 



^oConklin, 19050, 6, c, 1906, 1931; Dalcq, 1935, 1938; and various citations by these 

 authors. 



