52 TEXT-BOOK OF EMBRYOLOGY. 



median plane of the future embryo. Summing up, it may be stated that 

 there is a tendency in the frog for the median plane of the egg, the first 

 cleavage plane and the median plane of the embryo to coincide; but, remem- 

 bering that all these planes contain the egg axis, any other relation may be 

 encountered. 



On the surface the first cleavage furrow appears as a shallow groove on 

 the pigmented side of the egg and then gradually extends around to the yolk 

 pole. This is the surface indication of the division which separates the egg 

 into halves or blastomeres. If the cleavage plane coincides with the plane of 

 symmetry, the two blastomeres are symmetrical and the gray crescent is 

 divided symmetrically; otherwise the two blastomeres are asymmetrical in 

 internal structure. The division is total, but the two cells remain flattened 

 against each other in close contact. It should be noted also that the division 

 is retarded in the vegetal portion of the egg by the yolk globules in the cyto- 

 plasm. The retardation is so marked that the cleavage furrow of the second 

 division appears at the animal pole before the first furrow has reached the 

 vegetal pole. The second furrow crosses the first at right angles at the pig- 

 mented pole and extends around to the yolk pole in the same manner as the 

 first. The second cleavage plane, of which this second furrow is the surface 

 marking, intersects the first at right angles and thus divides each of the first 

 two blastomeres into equal parts. The direction of the plane is determined 

 by the position of the spindle in each primary blastomere, this lying in the 

 direction of the greatest cytoplasmic mass. The first four blastomeres are 

 approximately equal in size and contain equal amounts of cytoplasm and yolk. 

 They remain in close contact so that collectively they still form a sphere 

 which is marked on the surface by shallow grooves. 



The third cleavage planes intersect the first two at right angles but lie 

 nearer the animal pole than the vegetal pole, the furrow on the surface 

 appearing about 60 degrees from the animal pole. In this manner the four 

 blastomeres are divided into eight (Fig. 28, A). The upper four members 

 are smaller and contain an excess of cytoplasm while the lower four are 

 larger and contain an excess of yolk. This condition gives rise to the terms 

 micromeres and macromeres. In some instances the third cleavage plane 

 deviates from the latitudinal, even to being meridional, in one or more blasto- 

 meres. Typically the fourth cleavages are meridional, producing eight 

 micromeres and the same number of macromeres. Here again the planes may 

 deviate from the meridional position and disturb the typical pattern. Not 

 all the blastomeres necessarily divide at the same time, as might be implied 

 from the description. The lack of synchronism is especially true between 

 micromeres and macromeres because in the latter the process of division is 

 retarded to a marked degree by the inert yolk. From the fifth cleavage on, 





