596 WILSON. [ Vo L. V 1 1 1 . 



In the stage of the first gill-slit (Figs. 136-138) half-sized 

 dwarfs are rather rare, and most of them show a distinct ten- 

 dency towards an abnormal development of the tail region, 

 which is often thickened and distorted, and frequently bent 

 downwards posterior to the anus. Some of the full-sized 

 embryos, however, show a similar modification. A few of the 

 y^ larvae at this stage are perfectly developed in every respect ; 

 but, as Figs. 137, 138 show, the normal proportions of the 

 body are not quite maintained, the regions behind the ex- 

 panded anterior part being relatively shorter. The notochord 

 is relatively thicker, its diameter being nearly or quite equal 

 to that of the normal larva. The unique fourth-sized dwarf 

 observed at this stage {Fig. 139) is well developed in front, 

 showing a very distinct prae-oral pit and neuropore, but the 

 mouth and first gill-slit have not broken through. Posteri- 

 orly it is distinctly segmented, but shows two modifications : 

 (i) The intestine is wanting behind the enlargement, the 

 stomach ending blindly as shown in the figure ; (2) the tip 

 of the tail, including a portion of the notochord, is sharply 

 flexed toward the dorsal side and adherent to the body wall 

 in that region. 



Double embryos are extremely rare, and I have seen only 

 two specimens, neither of which was normally developed. In 

 the specimen shown (Fig. 140) the twins seem to be joined 

 by the ectoblast alone, and all the internal parts are distinct. 

 One of the individuals is nearly normal in structure. The 

 other is defective in the head region, and shows no prae-enteric 

 region.^ 



The foregoing facts leave no doubt that all the induced 

 forms of development show in general a lack of developmental 

 power that becomes more pronounced as the ontogeny ad- 

 vances. I have not determined whether the double embryos 

 ever split apart to form two separate twins, as Driesch 

 observed in Echinus. 



1 Mr. Willey, who has done so much to advance our knowledge of the later 

 stages of Amphioxus, informs me that he has observed free-swimming double 

 embryos at this stage in which the twins were joined back to back, and appeared 

 to be normally formed. 



