58 PROCEEDINGS OF THE AMERICAN ACADEMY 



one who ha3 undertaken a comprehensive comparison of these two 

 types of cleavage ;■** and he was the first to show that variations in the 

 cleavage of the frog's ovum occur which bring the two types together. 

 While Balfour and others have supposed that the '' equatorial " cleav- 

 age comes considerably later in the teleostean ovum than in the am- 

 plibian, and that therefore the order of the cleavage-planes is not the 

 same in the two types, Rauber holds that they occur only in the latter, 

 being replaced in the former by meridian cleavages (" Liiugsfurchen "). 

 It is very rare, according to Rauber, that we have true mei-idian 

 cleavages in the frog's ovum ; the cleavage-planes usually called 

 meridian do not seek the pole, but shun it. This is particularly true 

 of the third and fourth meridian cleavages, the polar distance of which 

 is often so great that they run nearly parallel to the first and second, 

 presenting thus the teleostean pattern. According to this view, the 

 third and fourth cleavage -planes of the teleostean ovum would corre- 

 spond to the fourth and sixth of the frog's ovum, the two equatorial 

 cleavages being skipped. The "Polflucht" theory of Rauber breaks 

 the homology of the cleavage-planes between the second and third 

 cleavage, and is in so far unsatisfactory. Balfour, in common with 

 most authorities, has mistaken the first concentric cleavage, which 

 occurs in passing from the 16-cell to the 32-cell stage, for the first 

 equatorial cleavage. 



We hold that the order of these cleavage-planes is identical in both 

 types ; and, accordingly, that the first, second, third, and fourth in 

 the one correspond exactly to the first, second, third, and fourth in 

 the other. "Polflucht" offers, to our thinking, no explanation of the 

 origin of " parallel " cleavages in the ovum of the teleost. The real 

 cause of an alternation fiom meridian to equatorial cleavage has never, 

 so far as we know, been stated. The equatorial plane of cleavage 

 is a forced one, and hence it follows the meridian cleavages. The 

 meridian planes are the natural planes of cleavage ; and the equatorial 

 only a dernier ressort, introduced in accommodation to the elongated 

 form of the blastomeres produced by meridian cleavage. The same is 

 true of the concentric cleavages. The cell must elouiiate in order to 

 divide; but it elongates in a vertical direction only when it is not free 

 to do so horizontally. In tlie blastodisc of the fish ovum the necessity 

 for a horizontal cleavage arises later than in the frog's ovum ; and it 

 naturally arises earlier in the central than in the marginal cells. The 



*^ Kauber. " Neue Grundleguiigen zur Kenutniss der Zelle." Morpli. Jahrb., 

 VIII., pp. 255-335, 1882. 



