No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 357 



by two meridionals. According to Kupffer (-90) either may 

 occur and from the third cleavage on no regularity exists. 

 Kupffer also adds a third order which is called tangential. 

 Comparing the variations in the cleavage of Amphibia and 

 Petromyzon we observe in the second and third acts of the 

 latter the irregularities manifested by the third and fourth of 

 the former. 



5. Cleavage of Coregonus. 



The observations were made largely on preserved material, 

 yet notes made three years ago on the cleavage of the living 

 egg have served as a guide. 



First cleavage. — The first furrow may divide the blastodisc 

 into two equal parts as in PI. XIX, Fig. 16, where they lie at 

 first closely together, but soon assume the condition indicated 

 in Fig. 1 7, in which they lie quite apart ; or they may later 

 become widely separated at either end as in Fig. 18. In case the 

 first furrow gives rise to two unequal blastomeres, which is the 

 usual occurrence, it is impossible to classify the variations, 

 since they range from almost equal size to a condition in which 

 one is two or three times the size of the other, as in Fig. 19. 



This inequality in size of the first two blastomeres is one 

 often observed in Teleosts. Rauber (-sa) observed it in Gobius, 

 Agassiz and Whitman ('84) in Ctenolabrus, Henneguy (-88) in 

 Salmo, and H. V. Wilson (-91) in Gadus, yet all produced nor- 

 mal embryos. 



Second cleavage. — The second set of furrows may form 

 right angles with the first, giving rise to four equal blasto- 

 meres (Fig. 22). They may pass from the same point at one 

 side of the pole, resulting in two smaller and two larger blasto- 

 meres (Fig. 24). Again the two furrows may depart from 

 points widely separated (Fig. 23) as in Ambly stoma and Petro- 

 myzon. If two unequal blastomeres are produced by the first 

 cleavage, a variation often occurs in which the smaller blasto- 

 mere remains undivided (Figs. 20 and 21) during this cleavage. 

 Again the larger may divide twice while the smaller undergoes 

 but a single division, as in Fig. 25, or the opposite may occur 

 (Fig. 26), although seldom observed. 



