No. 2.] EARLY DEVELOPMENT OF AMBLYSTOMA. 355 



period between the late blastula and the closure of the neural 

 folds. I have often observed this movement going on in a 

 large number of eggs at the same time, some rotating in one 

 direction, others in the opposite. 



Clarke states that in Amblystoma punctatum the surface of 

 the body is covered with cilia, at the time the neural folds 

 close, by means of which it keeps up its rotary motion. I 

 have endeavored to detect cilia by teasing in normal saline 

 solution, also by osmic acid fixation, but without success. 



4. Cleavage of Petromyzon. 

 In the study of this form I have been greatly assisted 

 through the kindness of Dr. Johnson, who has not only placed 

 his material at my disposal, but also has allowed me to use 

 some of his notes and drawings on cleavage. 



First c/eavage. — The first furrow (PI. XIX, Fig. i) appears 

 at the superior pole in from 4 to 5 hours after fertilization, 

 the ends may progress at a uniform rate or one may exceed 

 the other in rapidity. The furrow is completed 1 5 to 30 min. 

 later. In the majority of cases the cells are nearly equal, but 

 exceptions occur ; out of 57 eggs examined from different lots 

 of material 12 showed quite decided variations. One of these 

 variations is shown in Fig. 5. 



So far as I am aware, the only observation showing a 

 marked difference in the formation of the first furrow is that 

 of Calberla (-78), who found the first cleavage in a horizontal 

 plane, dividing the egg into two unequal parts. As this con- 

 dition has never since been observed, it is fair to suppose that 

 it occurs but rarely. 



Second cleavage. — The formation of the second furrow 

 begins in about i hr. 30 min. after the appearance of the 

 first, progresses in a manner quite similar, and forms with it 

 right angles. In most eggs a cross furrow is present ; it 

 may arise as in Amblystoma by the two furrows starting 

 from widely separated points, or may be the result of a 

 shifting of the blastomeres as described in Amphioxus by 

 E. B. Wilson. 



