Experimentally fused larvæ

Abstract

When the eggs of Toxopneustes variegatus were subjected to a 5/8 molecular NaCl, after the removal of the fertilization membrane, considerable numbers were subsequently fused together. I have counted as many as forty percentum, in the optimum solutions, of agglutinated and fused pairs, triplets, etc. Few of these reached the pluteus stage of development due to the early death of all fusions of more than three eggs, and to the large mortality of even the double embryos.

The plutei contain at least three characteristic tissues, namely, body wall, archenteron, skeleton. The first two of these behaved essentially as described by Driesch in various European species, and by the writer in the American species Arbacia punctulata; i. e., the body walls or the archentera of plutei derived from separate eggs were united either incompletely or so completely as to give little or no evidence of the original dual character of the larvæ.

It was supposed that the skeletal structures united in the same manner as the archentera. The evidence furnished by Toxopneustes however clearly shows that in this species at least no fusion of the skeletal parts occurred. Instead some very interesting changes took place which may be stated briefly as follows: One of the pair of fused larvæ developed normally in every detail, the other developed in nearly every instance, incompletely. An almost perfect series of fused larvæ were obtained in which the incomplete pluteus lacked more and more of the characteristic parts that constitute the perfect larval skeleton; and the order of their disappearance was in the reverse order of their appearance in ontogeny.

The union of the two larvæ, involved the approximation of their branched and complex skeletons, whose parts frequently overlapped but never fused.