PROCESS OF CLEAVAGE IN BATRACHIAN OVA. 



507 



to the ^equator, about opposite the junction of the superior and 

 middle third of the axis of the sphere, and this should be made 

 to cut its way completely through. By this means, proceeding 

 from the upper pole, the spherule is divided into four segments, 

 whilst the larger inferior portion of the sphere remains un- 

 divided, with the exception of the surface, where the two 

 meridianal constrictions indicate the subsequent divisions. 



The formation of these three furrows proceeds gradually. 

 It may occupy, at a temperature of 18 20 Cent. (64 68 

 Fahr.), from three to four hours from the period of the extrusion 

 of the egg from the body of the parent. 



Before a groove is definitely formed, the surface becomes 

 wrinkled, and then again smooth, and this change occurs several 

 times consecutively. From the principal furrows numerous 

 smaller secondary furrows proceed, which are only of a tran- 

 sitory nature. Reichert has described these secondary furrows 

 as the furrow crown (Faltenkranz), and Max Schultze* has 

 shown that they are the expression of the movement of the 

 germ. 



At the point where the first three furrows cut one another 

 (that is to say, in what would be the upper half of the egg 

 floating in water,) a cavity forms. From my own experience 

 I am unable to say whether this coincides with the nucleus 

 cavity or not. The cavity enlarges owing to the retraction 

 and rounding off of the opposed angles of the segments. 



The further process of the cleavage is limited chiefly to the 

 four upper segments. These become smaller by continual sub- 

 division, whilst the cavity increases in size, till ultimately a 

 spacious cavity is formed in the upper third of the ovum (F, 

 fig. 399), that may be best comprehended with regard to the 

 subsequent processes that take place in it by the following de- 

 scription. It may be regarded as an apple so excavated in the 

 upper third of its axis that only the rind remains. The lower 

 and larger section of the apple would then be solid, whilst 

 there would be a cavity in the upper segment that is only 

 surrounded by a thin cortex. 



We designate the cavity in the ovum of the Frog Baer'^ 



* De ovorum ranarum Segmentatione, 1863. 



