576 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tuitous external conditions. But how cleavage pattern is determined in a 

 spherical undeformed egg does not appear. If the cleavage axis is the dor- 

 siventral axis of the animal, the origin of the anteroposterior axis remains 

 obscure. The only evidence of a differential in the accounts of cleavage is 

 the somewhat less rapid division of the quadrant from which entoderm and 

 primitive germ cell originate. When blastomeres are killed or inhibited by 

 localized ultra-violet radiation but not removed, development of remain- 

 ing blastomeres is not altered, except for more or less overgrowth of the 

 undivided cell or cells by ectoderm (M. Jacobs, 1925). These experiments 

 give further evidence concerning relation of embryonic axes to cleavage 

 pattern, confirming and extending that from normal development. In 

 Cyclops and also in Polyphemus the germ path may be either in a right or in 

 a left quadrant, that is, asymmetry of cleavage pattern in this respect may 

 be either dextral or sinistral.'^ 



In eggs of various other entomostracans studied a cytoplasmic polarity 

 is distinguishable, either before or after maturation, and becomes the axis 

 of differentiation ; or this axis coincides with the axis indicated by polar- 

 body position."* In Polyphemus, however, first and second cleavages are 

 obHque to the egg axis, but this axis becomes the axis of differentiation 

 (Kiihn, 1912); but in Lepas, according to Bigelow (1902), there is a shift 

 of the egg axis in relation to the oblique first cleavage. According to these 

 data, there are considerable differences in cleavage pattern and in its re- 

 lations to egg pattern and to pattern of differentiation among the ento- 

 mostraca; some of the conclusions suggest that further investigation is 

 highly desirable. 



ASCIDIAN CLEAVAGE AND DEVELOPMENTAL PATTERN 



Cleavage pattern and developmental pattern of ascidians are apparent- 

 ly closely related. In the freshly extruded egg of Styela {^Cynthia) po- 

 larity is indicated only by position of the nucleus close to the egg surface, 

 the region in which it lies becoming the apical pole. Following extrusion 

 the nucleus breaks down, an apical area of clear cytoplasm appears, and 

 the first polar spindle forms in it. The egg also shows a surface-interior 

 pattern, an ectoplasm containing yellow pigment, and an entoplasm with 

 yolk (Conklin, 1905a). Whether the nuclear position near the surface is 

 determined by a pre-existing cytoplasmic polarity or is itself the determin- 

 es Kuhn, 1912; Fuchs, 1914; M. Jacobs, 1925. 



"■Grobben, 1879, 1881; Samassa, 1893; McClendon, 1906, 1907; Muller-Cale, 1913; 

 Kruger, 1922. 



