GENERAL CONSIDERATIONS 287 



right and left halves; it therefore achieves a sundering of the egg substances 

 along the future median plane of the embryo. The second cleavage plane 

 occurs at right angles to the first (Conklin, '05, a and b). This condition ap- 

 pears to be true of other ascidians, such as Ciona, Clavelina, etc. (Wilson, 

 E. B., '25, p. 1012). The behavior of the early cleavage planes is similar in 

 Amphioxus (Conklin, '32). Cleavage planes such as the foregoing, which 

 always divide the egg in a definite way have been described as "determinate 

 cleavage" (Conklin, 1897). Study figures 116, 132 and 167. 



The first cleavage plane in the eggs of some frogs (e.g., Rana jusca and 

 Rana pipiens) shows a great tendency to bisect the gray crescent and thus 

 divide the embryo into right and left halves. However, unlike Styela, and 

 Amphioxus, the first plane is not definitely fixed; considerable deviation may 

 occur in a certain percentage of cases in any particular batch of eggs. In the 

 newt, Triturus viridescens, the first cleavage plane generally is at right angles 

 to the median plane of the future embryo (Jordan, 1893). In Necturus 

 maculosus the first cleavage plane may in some eggs coincide with the median 

 plane of the embryo, while the second cleavage plane may agree with this 

 plane in other eggs. In some eggs there is no correspondence between the first 

 two cleavage planes and the median plane of the embryo; however, the planes 

 always cut from the animal to the vegetal pole of the egg (Eycleshymer, '04). 



In the teleost fish, Fundulus heteroclitiis, in the greater percentage of cases, 

 the long axis of the embryo tends to coincide with either the first or second 

 cleavage planes (Oppenheimer, '36). Other teleost fishes appear to be similar. 

 In the hen's egg the first cleavage plane may or may not lie in the future 

 median plane of the embryo (Olsen, '42). 



In some species it appears that the unfertilized egg may possess bilateral 

 symmetry. For example, in the frog, Rana jusca, the point of sperm entrance 

 evidently has an influence in orienting the plan of bilateral symmetry, and, 

 as a result, the gray crescent appears opposite the point of sperm contact with 

 the egg. However, in Rana esculenta and in Discoglossus pictus, two other 

 anuran species, there is no constant relationship between the point of sperm 

 entry and the plan of bilateral symmetry of the egg (Pasteels, '37, '38). In 

 the latter cases, unlike, that of Rana jusca, the stimulus of sperm entry pre- 

 sumably does not influence the plan of bilateral symmetry which is determined 

 previous to sperm entrance. 



It is to be noted that there is a strong tendency in many of the above species 

 for the first cleavage amphiaster to orient itself in such a manner as to coincide 

 with the median plane oj the embryo or to be at right angles to this plane. 

 This fact suggests that the first cleavage amphiaster is oriented in terms of 

 the egg's organization. It further suggests that the copulation paths of the 

 respective pronuclei, as they move toward each other, together with the re- 

 sulting cleavage path of the pronuclei (fig. 1 39), are conditioned by the inherent 

 organization of the egg's cytoplasm. 



