438 HOLMES. [Vol. XVI. 



individual variation (Watas^ '91), the early cleavage is quite 

 definite, but soon the divisions become too irregular to follow 

 in detail. In the tunicates we find bilateral cleavage of a 

 most conspicuous and determinate type, the median axis of the 

 embryo, as in the preceding forms, being marked out by the 

 first cleavage furrow (Seeliger, van Beneden and Julin, Castle, 

 '96). Among the vertebrates there are no cases of that regu- 

 larity and determinateness of cleavage which characterize the 

 early development of many invertebrates. The cleavage is 

 quite regular in some forms until a certain period, after which 

 the divisions follow no apparent order. In most forms whose 

 cleavage has been carefully studied there has been found con- 

 siderable variation in the cleavage of different eggs of the same 

 species. To the extent, however, that the cleavage of the eggs 

 of vertebrates follows any definite plan, it may be said to belong 

 to the bilateral type. The first cleavage plane often coincides 

 with the median axis of the embryo, though in some forms it 

 may apparently form any angle with this axis. In Amphioxus, 

 although the form of cleavage is exceedingly variable, exhibit- 

 ing cases of radial, spiral, and bilateral divisions in different 

 eggs, there is a predominant tendency to bilaterality in the 

 early cleavage (Wilson, '93), and the first cleavage marks the 

 future longitudinal axis. The cleavage of fishes, after the first 

 few divisions, usually becomes quite irregular. In Batrachus, 

 according to Miss Clapp, the first cleavage plane may form a 

 considerable angle with the median axis of the embryo. The 

 early cleavage of this form is, nevertheless, quite markedly 

 bilateral, though, as Miss Clapp informs me, it is subject to 

 considerable individual variation, and the plan of cleavage 

 becomes very irregular in later stages. Morgan found, also, 

 in Fundulus that, while the cleavage up to a certain stage was 

 of the bilateral type, the first cleavage bears no constant rela- 

 tion to embryonic axes. While in the frog and some other 

 Amphibia the first cleavage plane and the median axis of the 

 embryo commonly coincide, the first cleavage plane in Diemyc- 

 tylus (Jordan) and Triton (Hertwig) is usually transverse to this 

 axis. The axial relations of the first cleavage in case of the 

 frog, however, are known to be largely influenced by gravity 



