234 



Variations in Cleavage Patterns. 



One of the features of cleavage which throws much light on the 

 question under consideration is the variation in the position of the 

 cleavage grooves. The early writers on amphibian development de- 

 scribed and figured the cleavage as regular both in rhythm and pattern. 

 M. ScHULTZE was one of the first to point out the existence of wide 

 variations in the formation of the third meridionals. Rauber later 

 made a careful study of the earlier cleavage and he also found wide 

 variations ; so frequently did the first and second grooves pass widely 

 from the upper pole that he thought there must be some sort of 

 "Polflucht". Jordan and Eycleshtmer ('92) after a careful study 

 of the variations in a number of amphibia concluded that "irregular- 

 ities of cleavage have no appreciable effect upon any stage of de- 

 velopment of the embryo". The later observations by Grönroos ('90), 

 V. Ebner ('93), Morgan and Tsuda ('94), Kopsch ('00) and others 

 have likewise emphasized the significance of these variations. 



In studying the cleavage of the egg of Necturus I have been im- 

 pressed by the great irregularity in the formation of the various cleav- 

 age grooves and have represented, in Figs. 21 — 40, twenty eggs 

 taken from a typical nest containing sixty-two. While the first cleav- 

 age groove in Necturus is approximately vertical, and in the majority 

 of eggs approaches a meridional, I doubt whether a singly egg could 

 be found in which that groove would coincide precisely with any me- 

 ridian. In other words the cytoplasm is always unequally divided, 

 giving rise to blastomeres which in many instances are decidedly un- 

 equal (Figs. 22, 24, 25). If the median plane of the embryo coincides 

 with this groove we should expect to find a markedly unsyrametrical 

 embryo. These eggs, however, do not give rise to embryos or larvae 

 which vary in any particular, thus far discoverable, from those derived 

 from eggs which cleave in a more regular and symmetrical manner. 

 The two grooves which constitute the second cleavage begin as slight 

 indentations on either side of the first groove and usually extend in 

 vertical or meridional planes at right angles to the plane of the first 

 division. But instead of conforming to this general arrangement they 

 may extend in almost any direction, forming angles of varying degrees, 

 as shown in Figs. 22, 23, 24. It often happens that the points of origin 

 are widely separated, in which cases the deviations in this and suc- 

 ceeding cleavages are very striking. Again these two grooves may 

 depart from the same point giving the appearance of a single groove 

 as shown in Fig. 25. 



