276 



GENERAL EMBRYOLOGY 



meres form here, they are composed of this material. The 

 micromeres, which later form the mesenchyme, always appear 

 at the pole opposite the micropyle (Fig. 109), which marks the 

 point of attachment of the ovum in the ovarian germinal 

 epithelium; this is also the point at which gastrulation com- 

 mences. The centrifuge brings about a stratification of these 

 substances which is independent of the polarity of the ovum, 

 since the ovum may assume any position with reference to the 



FIG. 128. Normal cleavage in the sea-urchin, Arbacia, following abnormal 

 distribution of egg substances by centrifuging. From Morgan and Spooner. 

 The figures are turned so that the pigment (dotted area) is downward. The 

 location of the cleavage planes, and the position of the micromeres, which always 

 mark the invaginating pole also, are independent of the induced stratification of 

 the egg substances. 



axis of rotation of the machine. The pigmented protoplasm 

 may be thrown to any part of the cell. But Morgan has found 

 that the cleavage of eggs with abnormally distributed sub- 

 stances proceeds normally with reference to the original 

 polarity of the ovum and not according to the induced arrange- 

 ment. The micromeres, for example, continue to form 

 opposite the micropyle, and gastrulation occurs here, as usual, 

 although the pigmented protoplasm may occupy some remote 

 and unusual position in the cell (Fig. 128). Perfectly typical 

 larvae develop from such eggs, normal save in the distribution 

 of pigment. Development and differentiation thus seem to be 

 quite independent of the so-called " formative stuffs," which 

 are, in such instances, evidently not " organ forming." 



