geometrical interpretation, because the geometrical 

 regularity of distribution of the fissures is frequently 

 misrepresented without violation of the following notion 

 of development. Further Baer established the following 

 general rule: "... If in the separated yolk mass (or 

 blastomers, as they are now called — L.B.) one side is 

 significantly longer than the other, then it also under- 

 goes division. According to this rule, the equatorial 

 regions must be divided by vertical fissures, and the 

 circumpolar regions by horizontal fissures. However, 

 deviations from this rule are possible" (p. 499) . 



After more than forty years, the established 

 regularity of the axis of division was identified as 

 Hertwig's law, 22 while, in fact, it must be called Baer's 

 law. 



The succession of the planes of division in the frog 

 ovum Baer explained in the following way. The first 

 fissure, beginning from the dark pole, is meridional. It 

 divides the ovum into two parts with identical vertical 

 and horizontal diameters; that is, at these parts, accord- 

 ing to Baer's law, the second division in vertical and 

 horizontal planes is equally possible. However, it always 

 takes place only in the vertical plane and it, as the first, 

 also begins from the dark pole. In the four parts of the 

 ovum, the vertical diameter is twice greater than the 

 horizontal; they are divided in accordance with Baer's law, 

 in the equational plane. Then again eight blastomeres 

 ("yolk masses") appear. Every one of them is bound on 

 three sides by equal planes, and on the fourth side by an 

 identical part at all the spherical surfaces. Here again 

 it would not seem to matter in which plane the division of 

 these parts of the divided ovum takes place. The division 

 regularly begins from the dark pole, however, and the 

 fissures of division appear again meriodionally. Later, 

 strict regularity of division is lost, but the tendency to 



12. It is known under this name everywhere, including the 

 Russian handbooks (see for example A. Maksimov, 

 OSNOVY GISTOLOGII (THE BASIS OF HISTORY), 

 Part 1:UCHENIE KLETKE (Study of the cell) 

 (1917) , p. 334. 



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