IV. i INITIAL STRUCTURE OF THE GERM 



173 



Samassa has, however, shown that a short archenteron with 

 dorsal lip, tog-ether with traces of notochord and mesoderm, may 

 be formed from the four animal cells alone, the dead yolk- cells 

 making a floor to the archenteric cavity and protruding as 

 a large yolk-plug; and Morgan has obtained a similar result 

 with the four vegetative cells alone. 



From all these investigations it seems reasonable to infer that 

 each of the first two blastomeres of the Frog's egg may under 

 certain circumstances acquire the polarity of a whole, and be 

 capable of giving rise to an embryo whose complete development 

 is only prevented by the impediment offered by the presence of 

 the other, whether living or dead. Were it possible to com- 

 pletely separate the two blastomeres, we may surmise that each 

 would become a perfect embryo : a surmise which is raised to 

 a certainty by our knowledge of 

 what happens in the newt. For 

 in this form it is possible to 

 separate the two cells by means 

 of a noose of fine hair tied round 

 the egg in the plane of the first 

 furrow, as Herlitzka has shown, 

 and in this case each half seg- 

 ments as a whole and develops 

 into a whole larva of rather more 

 than half-size (Fig. 87). Her- 

 litzka has further investigated 

 the dimensions of the organs in these embryos ; those of the me- 

 dulla and notochord he finds are the same as in embryos developed 

 from a whole egg, those of myotomes and gut a little less. The 

 size of the nuclei and cells in the medulla and myotomes is the 

 same as in the whole (^) embryo 1 ; the number of nuclei in the 

 medulla is the same, in the myotomes one-half. He concludes 

 that some organs need a certain minimum number of cells for their 

 differentiation, while the cells must attain to certain dimensions. 



1 It will be convenient henceforward to designate the whole egg, or 

 the embryo or larva developed from it, by the symbol $, each of the first 

 two blastomeres and the embryo or larva, whether complete or incom- 

 plete, formed from it by \, and so on ; thus a embryo means one which 

 arises from three out of the first four blastomeres. 



FIG. 87. Egg of the newt 

 (Triton cristatus), with two com- 

 pletely normal embryos, obtained 

 by tying a thread (sf) round the 

 egg in the first furrow, g, jelly 

 membrane. (After Herlitzka, from 

 Korschelt and Heider.) 



