EMBRYOLOGY OF CRYPTOBRANCHUS 463 



out the late cleavage stages, and in some eggs enables one to 

 identify the early cleavage furrows even after the beginning of 

 gastrulation. 



Deviations from the type in Stage 3 show a series of conditions 

 connecting the typical one with a true latitudinal third cleavage. 

 In such cases the third cleavage furrow proceeds more obliquely, 

 and at an earlier stage joins the first nearer the animal pole (figs. 

 68 to 72). In some cases one or more of the third cleavage fur- 

 rows are truly latitudinal (figs. 70 to 72). 



Rare cases occur in which a third cleavage furrow originates 

 at the animal pole, or from a first cleavage furrow (see fig. 67 

 for an example of the latter case) ; occasionally, a third cleavage 

 furrow may reach the vegetal pole, or unite with a second cleavage 

 furrow near the vegetal pole (figs. 94 and 95). 



In comparing the third cleavage pattern of Cryptobranchus 

 with that of other forms, one of the most obvious generalizations 

 brought out is that a vertical third cleavage is characteristic of 

 heavil}' yolk-laden and highly telolecithal eggs: e.g., the squid 

 (Watase '91); Amia (Dean '96, Whitman and Eycleshymer '97); 

 Lepidosteus (Dean '95, Eycleshymer '99); Acipenser (Dean 

 '95); Ctenolabrus (Agassiz and Whitman '84); Serranus (H. V. 

 Wilson '91); Ceratodus (Semon '00 and '01); Lepidosiren (Kerr 

 '00 and '09); Cryptobranchus japonicus (deBussy '04 and '05); 

 and the pigeon (Blount '07). But the rule is not absolute; con- 

 cerning the third cleavage of Necturus, Eycleshymer ('04) says: 

 "In most cases the cleavage grooves are irregularly formed and 

 it might be said that the variations are so numerous and so di- 

 verse that a special description must be written for each egg." 

 From this statement and an inspection of his figures (see also 

 Eycleshymer and Wilson '10), it appears that a type cannot be 

 recognized for the thii d cleavage of this egg ; that the irregularity 

 is greater than in the case of Cryptobranchus and that there is 

 a more marked tendency for the third cleavage furrows to come 

 in latitudinally. My material for the very early cleavage stages 

 of Necturus is too scanty to enable me to form any conclusion 

 based on direct observa;tions, but some unpublished figures of 

 the early cleavage of Necturus drawn from the living egg by 



