BIOLOGY OF EGGS AND IMPLANTATION 



847 



Again cleavage may be either partially 

 or completely inhibited by the addition of 

 colchicine to a culture medium containing 

 the fertilized eggs of frogs (Samartino and 

 Rugh, 1946) or rabbits (Pincus and Wad- 

 dington, 1939). A similar effect is observed 

 when this drug is injected into mated mice 

 (Waldo and Wimsatt, 1945). The rate of 

 cleavage also depends on the amount of 

 stored yolk. This is particularly true in the 

 macrolecithal eggs of frogs and birds. 



The peculiar phenomenon of deutoplas- 

 molysis, or extrusion of yolk from ferti- 

 lized and cleaving eggs, has been described 

 in the bat (Van der Stricht, 1909) , various 

 marsupials (Hartman, 1928), the horse 

 (Hamilton and Day, 1945), the guinea pig 

 (Lams, 1913), the cat (Hill and Tribe, 

 1924), the pig (Heuser and Streeter, 1929), 

 and the ferret (Hamilton, 1934). In the 

 cleaving eggs of the horse, a large amount 

 of the yolk is extruded into the perivitelline 

 space. The significance of this process is un- 

 known, but it has been suggested that elimi- 

 nation of yolk may be necessary in order to 

 establish a normal nucleocytoplasmic ratio 

 (Levi, 1915). 



Except for the monotremes, all mammals 

 have meiolecithal eggs which undergo a 

 complete or holoblastic type of cleavage. A 

 discrepancy in the size of the first two 

 blastomeres has been reported for a number 

 of mammals and seems to be the usual con- 

 dition (see Amoroso, Griffiths and Hamil- 

 ton, 1942, for review of this subject). 



The second cleavage division occurs in 

 two planes at right angles to each other. Di- 

 vision of the two blastomeres is not neces- 

 sarily synchronous and accounts for the 

 frequent observation of a 3-cell stage. In 

 an ovum containing two blastomeres of un- 

 equal size, the larger cell apparently has 

 some priority in the next two divisions and 

 this probably explains the origin of eggs 

 containing an unequal number of cells. In 

 the 4-cell stage, the blastomeres are ar- 

 ranged in the form of a tetrahedron, due to 

 the preceding orientation of the two mitotic 

 spindles at right angles to each other. Dif- 

 ferences in the size of the blastomeres have 

 been recorded in almost every species. 



By the end of the 16-cell stage, several 

 of the blastomeres have been moved cen- 



trally thus forming the morula. In subse- 

 quent cleavages, the smaller, peripheral 

 cells divide more rapidly and an asyn- 

 chrony, already present, is accentuated. 

 Then fluids begin to accumulate between 

 peripheral and central cells, giving rise to 

 the cavity of the blastocyst. 



During cleavage there is a significant 

 diminution in the volume of the total 

 ooplasm. In the first cleavage division of 

 the monkey {Macacus rhesus), Lewis and 

 Hartman (1933) recorded a shrinkage of 

 44 per cent. During the 1-cell stage of the 

 mouse, Lewis and Wright (1935) noted 

 shrinkage of as much as 25 per cent with a 

 further decrease in volume as cleavage con- 

 tinued. The hamster egg is even more re- 

 markable for the very large volume of its 

 perivitelline space (Austin, 1957). 



As mammalian ova of various species are 

 studied, attention is being directed to the 

 dift'erences in the size of blastomeres and 

 rate of cleavage in the hope of finding evi- 

 dence for the sorting and localization of 

 specific determining substances in the zy- 

 gote. It has been suggested that in the eggs 

 of the monkey (Lewis and Hartman, 1941), 

 pig (Heuser and Streeter, 1929), goat (Amo- 

 roso, Griffiths and Hamilton, 1942) , rabbit 

 (Van Beneden, 1875), and mouse (Sobotta, 

 1924), the more rapidly dividing blasto- 

 meres are the precursors of the trophoblast 

 and the more slowly cleaving cells the pre- 

 cursors of the inner cell mass or the embryo 

 proper. 



Even though discrepancies in the size of 

 the first two blastomeres have been de- 

 scribed in many mammals, there is at yet 

 little evidence of a qualitative difference be- 

 tween them. Heuser and Streeter (1929) 

 could not find a demonstrable cytologic dif- 

 ference between the first two blastomeres 

 of the pig. Hamilton (1934) suggested that, 

 at least in the ferret, size differences of the 

 blastomeres can be explained by the chance 

 division of the cytoplasm in the first cleav- 

 age. Despite the difference in size of the 

 first two blastomeres in the mouse, Gresson 



(1941) has shown that the mitochondria 

 are equally divided between them. Further- 

 more, the observations of Nicholas and Hall 



(1942) do not support the theory of abso- 

 lute determination of the early blastomeres. 



