BY DR. KLEIN. 159 



and so on, until the whole germ is divided into a number of 

 globules, each of which consists of protoplasm inclosing a 

 vesicular nucleus, and, like the entire germ, is endowed with 

 the capability of performing amoeboid movements. These 

 cleavage globules are called "embryo cells." Onlv the germ 

 or blastoderm takes part in the cleavage, since this alone is 

 endowed with amo3boid movement. Consequently in mero- 

 blastic eggs the cleavage is said to be partial. In the holo- 

 blastic, on the other hand, the whole egg divides, for the whole 

 is germ ; it is, therefore, said to exhibit total cleavage. 



Study of the Process of Cleavage in the Ova of 

 Fish and Amphibia. The cleavage process should be 

 studied, in the first place, in the entire ovum ; the knowledge 

 thus gained being completed by sections of the germ at the 

 cleavage time. Of meroblastic eggs, those of the trout are 

 best suited for this stud} r . Several such eggs are examined 

 under the microscope in a watch-glass, in the water in which 

 the}' have lain since undergoing fertilization, strong trans- 

 mitted light and a weak magnifying power (90-100) being 

 employed (see figs. 159-163). At the tenth hour after fertiliza- 

 tion, the blastoderm appears, lying upon the yolk like a lid 

 over a saucer-shaped depression ; the yolk, which forms the 

 bottom of this cavity, contains closely packed oil globules, 

 which have become aggregated at this pole of the yolk since 

 the time of fertilization. In the blastoderm amoeboid move- 

 ments are observable. About the twelfth hour, the first cleav- 

 age line appears. About the twenty-seventh, almost all the 

 eggs show two cleavage lines crossing each other. Between 

 this time and the end of the second day, eight segments may 

 be distinguished ; so that four cleavage lines are now seen on 

 the surface of the blastoderm. At the end of the seventh 

 day the process of cleavage has progressed so far that the 

 surface of the blastoderm appears beset with a number of 

 bosses, like a mulberry. The cleavage process is far more 

 easily studied in the holoblastic eggs of amphibia. If eggs of 

 the frog or toad, freshly spawned, are placed under the micro- 

 scope, in a small cell, which may be convenient!} 7 prepared upon 

 a slide by means of putty, it is seen (especially in the case of 

 the latter, where they are placed one behind the other in rows 

 in gelatinous strings), that only a very few are spherical: 

 generally one part of the surface is flattened : so that it 

 frequently happens that, in a long row of eggs, alternating 

 conical ones are met with. About the sixth or seventh hour 

 after spawning, it can be seen by transmitted light that most 

 of the eggs have become round. As this period of time ap- 

 proaches, the amoeboid movement of the germ becomes more 

 distinctly visible, presenting the appearance of an oscillation 

 at some point or other within the vitelline membrane. This 



