130 DEVELOPMENT OF THE FROG'S EGG [Cn. XII 



as though it were still in contact with its fellow-blastoim ic 

 When the organs appear in the larva, only half the full num- 

 ber of rows of swimming-paddles appear. Each row, however, 

 has its full complement of paddles. The invagination of ecto- 

 derm to form the " stomach" is very excentric in the half -larva, 

 but forms a closed tube running from the mouth-opening to the 

 excentric sense-plate. In several respects, therefore, the larva? 

 were distinctly half-larvte. But in another respect they were 

 more than half-larvae. The endodermal cells of the normal 

 larva arrange themselves into four hollow pouches, and the 

 " stomach " invagination passes in the central line of the four 

 pouches. In the half -larva, on the contrary, the endodermal 

 mass forms more than two pouches (i.e. more than half the 

 normal number in the whole larva). Two distinct pouches 

 are present and in addition, generally, a third smaller pouch is 

 formed. The latter lies excentrically. In the meeting-point 

 of the three pouches is the excentric " stomach " invagination. 



The isolated one-fourth blastomere segments also as a part 

 of a whole, and develops in some cases into a one-fourth larva, 

 having only two roivs of paddles (i.e. one-fourth the normal num- 

 ber), but with two endodermal pouches (i.e. with one more than 

 one-fourth the normal number). The three-fourth embryos 

 develop six rows of paddles (i.e. three-fourths of the normal 

 number) and four endodermal pouches. The problem is here 

 a complicated one, for while in one set of organs we find a half- 

 development, in other organs we find more than a half, but yet 

 not the whole development. 



The results show, however, beyond question, that, even when 

 isolated from its fellow, the one-half blastomere may give rise 

 to a larva that is in many respects only one-half of the normal 

 larva. 



There is yet to be described another series of experiments 

 that have a direct bearing on the interpretation of the preced- 

 ing results. Roux showed that if a part of the protoplasm be 

 removed from the unsegmented frog's egg, the egg may continue 

 in many cases to develop into a normal embryo. The eggs of 

 the sea-urchin lend themselves much more readily to this ex- 

 periment. They may be broken up into fragments of all sizes 



