EMBRYOLOGY OF CRYPTOBRANCHUS 469 



The biradial pattern of cleavage is thus preserved in the lower 

 hemisphere, and throughout the later cleavage stages affords 

 a trustworthy means of distinguishing first and second cleavage 

 furrows in this region. 



The fate of the fourth cleavage furrows that proceed vertically 

 must be studied in later stages. They usually join the second 

 furrow before reaching the lower pole (figs. 92 and 96). 



In a given egg the micromeres vary somewhat in size; but a 

 comparison of seventeen carefully drawn camera figures, and 

 the examination of a large number of additional eggs, lead to 

 the conclusion that in this stage there is no regularity in the 

 distribution of large and small cells among the micromeres. 



DeBussy's ('04) single figure of the fourth cleavage stage 

 of Cryptobranchus japonicus shows six miciomeres surrounded 

 by an approximately circular cleavage furrow, and two recent 

 furrows extending for a short distance vertically. 



In Desmognathus, according to Wilder ('04), the fourth cleav- 

 age is latitudinal ; this conclusion was based on the study of mate- 

 rial very limited in amount. Hilton ('09) states that in a large 

 number of eggs he has found only a few which exhibit so regular 

 a type of cleavage as described in Wilder's eight cell and later 

 stages. 



In Hynobius (Kunitomo '10), the fourth cleavage furrows are 

 more uniformly latitudinal than in Cryptobranchus allegheniensis. 

 In Necturus (Eycleshymer '04; Eycleshymer and Wilson '10) 

 and Diemyctylus (Jordan '93) a type is no longer recognizable. 



In Ceratodus (Semon '00 and '01) the fourth cleavage is lati- 

 tudinal. Amia (Dean '96; Whitman and Eycleshymer '97) 

 and Lepidosteus (Dean '95; Eycleshymer '99) resemble the type 

 with four micromeres described for Cryptobranchus alleghe- 

 niensis. 



Stage 5: (figs. 85 to 96; 210 and 211. This stage is reached 

 about four hours later than Stage 4. It is characterized by 

 the presence of the fifth cleavage furrows, giving a maximum of 

 thirty- two cells, some incompletely divided. More than half 

 of these cells are micromeres. 



