662 PATTERNS AND PROBLEMS OF DEVELOPMENT 



happens to be pushed by the accumulation of substance during growth 

 and that, if a polarity is determined by the differential between free and 

 attached pole, it is more or less completely obliterated by the activation 

 associated with polar-body formation. At the time of maturation a dis- 

 tinct dye-reduction gradient, decreasing from the region of polar-body 

 formation, appears, primarily in the egg cortex (Child, 1936a). 



Regional ovarian attachment of the oocyte, often with development of 

 a peduncle, is characteristic of many other animals, perhaps most com- 

 monly among mollusks and annelids; and, as far as the relation has been 

 determined, the free pole becomes apical in most of these." 



Oocytes of A scar is megalocephala are regionally attached to a rhachis 

 extending through the tubular ovary, the attached pole being more or 

 less pointed, the free pole rounded; but they usually become spherical 

 before maturation, and attached and free poles are not distinguishable. 

 Moreover, position of the second polar body, the only one adhering to the 

 egg surface, varies in relation to plane of cleavage, whether because posi- 

 tion of formation is variable or because its position changes after forma- 

 tion. Also, giant eggs formed by fusion of two may develop normally. 

 Zur Strassen (1906) and Boveri (1910a, b) believed that polarity in these 

 eggs is finally established late, after maturation or shortly before first 

 cleavage. Recently, however, it has been found that in certain eggs with 

 pear-shaped shells, presumably retaining the form of the ovarian oocyte, 

 polar-body formation usually occurs at the blunt pole, the free pole of the 

 oocyte, but may occur elsewhere. Moreover, the region of polar-body for- 

 mation, even when not at the free pole of the oocyte, may apparently 

 sometimes become the animal pole. These observations suggest that the 

 free pole normally becomes the animal pole and determines polar-body 

 formation there but that, when presumably abnormal conditions deter- 

 mine polar-body formation elsewhere, the ovarian polarity may be ob- 

 literated and a new polarity determined in relation to maturation.'^ 



A multicellular peduncle develops from epithelial cells of the ovary in 

 Limulus (Munson, 1898) and in arachnids (Balbiani, 1873). According 

 to Conklin (1932), the oocyte of Amphioxus and probably the ascidian 

 oocyte are attached by the pole which becomes apical or animal. 



'2 E.g., the free pole is apical in the nemertean Cerehratulus (C. B. Wilson, 1899; E. B. 

 Wilson, 1903); in the mollusks Cyclas (Stauffacher, 1893), Unio (F. R. Lillie, 1895), 

 Musculium (Okada, 1935), and Dentalimn (E. B. Wilson, 1904); in the annelid Chaetopterus 

 (F. R. Lillie, 1906), and in Sagitta (Stevens, 1904). 



'3 Schleip, 1924; 1929, pp. 230-33. See also Boveri, 19106. 



