464 BERTRAM G. SMITH 



Prof. Bashford Dean give confirmatory evidence of the irreg- 

 ularity of the third cleavage furrows. In Desmognathus the 

 third cleavage furrows were vertical and regular in the few eggs 

 studied by Wilder ('04); the third furrows depart from an earlier 

 cleavage furrow at some distance from the animal pole. But 

 Hilton ('04 and '09), who examined a considerable number of 

 eggs of Desmognathus in this stage, states that this regular and 

 vertical form of cleavage occurred in only two or three eggs; 

 in the others the third cleavage was irregular. In Diemyctylus 

 (Jordan '93) there is still greater irregularity in the third cleavage 

 than is recorded for Necturus or Desmognathus: "With the com- 

 pletion of the second furrow all consistent regularity is at an 

 end." 



In eggs less heavily yolk-laden, as in Amblystoma (Eycleshy- 

 mer '95) and the frog, the third cleavage is latitudinal. 



Especially interesting from a comparative point of view are 

 Budgett's observations on the cleavage of the crossopterygian 

 Polypterus as given by Kerr ('07). "From Budgett's pen 

 and ink sketch .... we can see that the segmentation 

 is at first characterized by its almost absolutely equal character. 

 We may infer with considerable certainty that the two merid- 

 ional furrows are succeeded by a latitudinal one which is prac- 

 tically equatorial." The egg of Polypterus is small, having a 

 diameter of a little over one millimeter. 



In urodeles we find a condition intermediate between the ver- 

 tical third cleavage characteristic of the fishes generally, and 

 the latitudinal third cleavage of the anura. In Cryptobranchus 

 the vertical type prevails; in Desmognathus, Necturus and Die- 

 myctylus there is increasing irregularity; in Amblystoma the 

 third cleavage is latitudinal. The possible phylogenetic signifi- 

 cance of the cleavage of Crytpobranchus will be considered later. 



As a rather general rule, in eggs in which the third cleavage 

 is usually vertical, the third furrows depart from the second 

 rather than from the first or from the animal pole. As has 

 already been seen in the case of Cryptobranchus allegheniensis, 

 this rule is by no means absolute; but in general it applies also 

 to the squid, and to the teleosts (e.g., Ctenolabrus, Serranus). 

 DeBussy ('04 and '05) has described the cleavage stages of 



