120 



THE AMERICAN NA TURALIST. [\'<)i.. XXXV 1 1 1. 



in the formation of an eight-cell stage, which is bilateral along 

 the plane of the first cleavage, and consists of the four cells A. 

 B. C. and D. upon one side, and of E. F. G. and H. on the 

 other (fourth row of Fig. i). These eight meridionally arranged 

 blastomeres do not seem to have equal value in respect to size, 

 for through a slight obliquity in these four new lines of cleavage 

 there result four smaller blastomeres which alternate with four 

 larger ones, and of these the latter alone usually attain the lower 

 pole while the former ones do not reach it. This is seen by a 

 study of the lower pole views in the fifth and sixth rows of Fig. 

 I, where the smaller blastomeres A. C. E. and G. intrude like 

 wedges along the lines of the first two cleavages, but do not 

 reach the pole save in the single instance of A. in the fifth row, 

 an unusual condition. 



The next cleavage is an horizontal one, forming an approxi- 

 mate circle about the upper pole, and cutting off small segments 

 from each of the eight blastomeres of the preceding stage (fifth 

 row). This results in a i6-cell stage, consisting of eight micro- 

 meres, a, b, c, etc., clustered about the upper pole, and eight 

 macromeres, the remaining parts of the original cells. This last 

 cleavage takes place so far above the equator of the egg that it 

 does not change the aspect of the lower half, and thus the 

 drawing of this egg (the third of the fifth row) would answer 

 equally well for the preceding one. 



By a comparison with Eycleshymer's studies of Amblystoma, 

 {Journal of Morphology, Vol. X, 1895) it becomes evident that 

 this latter cleavage is the one which is described as typically the 

 third in Amphibian eggs, and that the four meridional cleavage- 

 lines which result in the formation of the 8 cell stage, together 

 form the usual fourth. Indeed, this transposition of the two 

 cleavages occasionally occurs in Amblystoma, and as my obser- 

 \ation rests upon the study of but two eggs, it cannot be 

 asserted that the order described is the typical one in Desmog- 

 nathus. It is, however, identical with the method shown and 

 hgurcil by Kerr in Lepidosiren, and his figures of the third 

 cleavage (by means .)f the four short lines) as copied by Ziegler 

 (Figs. 213-314 ,,f his Enti.'icklnngsgcs. dcr nicderen Tien. 

 1902) would serve in every respect a.s better pictures of Des- 



