30 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



ve difference in the cytoplasmic substances which enter into the various 

 lastomeres becomes evident. 



That the presence or absence of a specific ooplasmic substance within the 

 lastomeres determines a difference in the developmental history of the cell 

 r cells has been shown experimentally for many animal species. For example, 

 1 the amphibian embryo it has been demonstrated both by constriction of 

 le developing egg and its membranes with hair loops (Spemann, '02, '03) 

 nd by placing a small glass rod in the cleavage furrow after the egg mem- 

 ranes have been removed (Ruud, '25) that each of the blastomeres of the 

 vo-cell stage will develop a complete embryo // the first cleavage plane bi- 

 dets the gray crescent. If, on the other hand, the first cleavage plane is at 

 glu angles to the median plane of the embryo, the blastomere which contains 

 le substance of the gray crescent will develop a complete embryo, whereas 

 le other one will give origin to a very imperfect form which does not gas- 

 ulate normally or produce a semblance of a normal embryo. 



Similar experiments upon the egg of the newt, Triton pahnatus, indicate 

 lat a marked difference in the "developmental potencies exists between the 

 orsal and ventral sides of the egg within a few minutes from fertilization, 

 he formation of the gray crescent seems to be a secondary phenomenon 

 hich makes this difference clearly visible in the eggs of some species" 

 Fankhauser, '48, p. 694). 



In Amphioxus, similar evidence is obtained after the blastomeres have been 

 lechanically isolated. Typical embryos are developed always from the first 

 vo blastomeres, for unlike the frog or newt, the first cleavage plane con- 

 \stently furrows the median axis of the embryo. These twin embryos are half 

 le normal size (Wilson, E. B., 1893; Conklin, '33). Right and left halves 

 f the four-cell stage also give rise to normal larvae. Moreover, blastulae also 

 evelop from isolated blastomeres of the eight-cell stage, bitt the blastulae 

 hich develop from the micromeres are smaller and have only one type of 

 ell, namely, ectoderm, and they never go further than the blastular stage. 

 )n the other hand, those from the macromeres are larger and have entoderm, 

 lesoderm, as well as ectoderm, but they never progress further than the 

 astrular stage of development (Conklin, '33). Reference should be made to 

 gure 167B in this connection. It is to be observed that the macromeres contain 

 otential mesodermal, entodermal, and ectodermal ooplasm, whereas the mi- 

 romeres lack the mesodermal and entodermal substances and contain only 

 ctodermal material. 



In the protochordate, Styela, a somewhat different condition is found. If 

 le cleaving egg of this species is separated at the two-cell stage into two 

 eparate blastomeres, each blastomere develops only one half of an embryo 

 Conklin, '05b, '06). That is, the right blastomere develops an embryo minus 

 he left half, while the left blastomere produces the opposite condition. There 

 s some tendency to develop or regulate into a complete embryo in that the 



