288 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



B. Types of Cleavage in the Phylum Chordata 



Cleavage in the phylum Chordata often is classified as either holoblastic or 

 meroblastic. These terms serve a general approach to the subject but fail to 

 portray the varieties and problems of cleavage which one finds within the 

 phylum. Under more careful scrutiny, three main categories of cleavage types 

 appear with typical holoblastic cleavage occupying one extreme and typical 

 meroblastic cleavage the other, while between these two are many examples 

 of atypical or transitional cleavage types. Moreover, the phenomena of cleav- 

 age are variable, and while we may list the typical cleavage of any one species 

 as holoblastic, transitional or meroblastic, under certain modifying circum- 

 stances the cleavage pattern may be caused to vary. 



Holoblastic cleavage is characterized by the fact that the cleavage furrows 

 bisect the entire egg. In meroblastic cleavage, on the other hand, the disc of 

 protoplasm at the animal pole only is affected, and the cleavage furrows cut 

 through this disc superficially or almost entirely. Superficial cleavage occurs 

 typically in certain invertebrate forms, particularly among the Insecta. How- 

 ever, in a sense, the very early cleavages in elasmobranch fishes, certain teleost 

 fishes, and in birds may be regarded as a kind of superficial cleavage. 



1. Typical Holoblastic Cleavage 



In typical holoblastic cleavage, the first cleavage plane bisects both poles 

 of the egg along the median egg axis, that is, the first plane of cleavage is 

 meridional. The second cleavage plane is similar but at right angles to the 

 first, thereby dividing the "germ" into four approximately equal blastomeres. 

 (See Sachs' rule (a), p. 286.) The third cleavage plane in typical holoblastic 

 cleavage occurs at right angles to the median axis of the egg and the foregoing 

 two meridional planes. (See Sachs' rule (b), p. 286.) As it does not cut along 

 the equatorial plane, but nearer the animal pole, it is described as a latitudinal 

 cleavage. Two meridional cleavage planes (see definition, p. 283) followed by 

 a latitudinal plane (see definition, p. 284) is the cleavage sequence charac- 

 teristic oj the first three cleavage planes of typical holoblastic cleavage. The 

 following chordate species exemplify typical holoblastic cleavage: 



a. Amphioxns 



In this cephalochordate there exists as typical a form of holoblastic cleavage 

 as is found anywhere in the phylum Chordata. The process of cleavage or 

 segmentation in Amphioxns has been described in the studies of four different 

 men; as such, these descriptions form four of the classics of embryonic study. 

 These studies were made by Kowalewski in 1867; Hatschek in 1881, English 

 translation, 1893; Cerfontaine, '06; and Conklin, '32. With the exception 

 of certain slight errors of observation and interpretation, Hatschek's work is 

 a masterpiece. 



