Experimentally fused embryos with special reference to giant larvæ formation,changes of symmetry,and changes of synchrony

Experimentally fused groups of two or more eggs of the seaurchin Arbacia punctulata were studied individually from the blastula stage through the larval stage as late as the 14-day larvae. These isolated groups were studied with respect to the behavior of the three major tissues, body wall, gut and skeleton, and, of the processes taking place I wish to mention briefly only three.

1. Contrary to the views of Boveri and DeHaan two fused eggs may develop into a single giant larva even when the axes and symmetry of the two eggs or blastula or gastrula are not in the position of two blastomeres of an egg. A considerable number of fused pairs of eggs were followed through their entire development, in which the axes of the two numbers diverged 35 to 135 degrees from each other, yet these eggs gave rise to single giant larvæ.

Some of the processes involved in the transformation of two asymmetrically fused eggs include (1) change of symmetry, (2) retardation, (3) repression of one of the members, (4) absorption of one or more parts, (5) conflict of the skeleton centers, (6) size and rate factors in development.

2. There is a definite tendency for the two members to grow unequally, the one becoming increasingly small, though the rate of development is little or not affected. The law of synchrony as developed by recent investigators certainly does not apply in these grafts, and the regulative changes are due in largest part to the other factors enumerated.

3. Much evidence was collected that an absolute and relative change of axes takes place in many instances and in two directions, towards a symmetrical arrangement of the two members and, away from such symmetry, and the extent of such changes was determined and measured.