Abnormal Development of Toad Ova 19 



due to abnormal distension of various regions with fluid. The 

 external form meanwhile became most irregular. This irregularity 

 of external form is more easily illustrated than described. See out- 

 lines on Plates II to V. At the period when the sketches were 

 made of the larvae here illustrated the control larvae had the ex- 

 ternal form shown in the outline on Plate I. The rudiments of 

 the hind legs were beginning to be externally visible. 



The organs of the experiment larvae show the same irregularity 

 of form as that shown by the body as a whole. Wax model 

 reconstructions of the organs of each individual larva which has 

 been preserved and of a series of stages of the normal larvae of the 

 toad would be necessary for a thorough study of the abnormalities 

 which exist in the experiment larvae. It has not seemed worth 

 while to make so elaborate a study because each experiment larva 

 is affected in such an individual way that it seems improbable that 

 broader generalizations could be drawn from such a study than 

 from the simple inspection of the serial sections of the larvae. 



Control Larva of the Size of the Experiment Larvae 



The control larvae at the period when the experiment larvae 

 began to show marked abnormalities in external form were about at 

 the stage described by Marshall in his Vertebrate Embryology as 

 characteristic of the tadpole of the frog shortly after hatching. 

 The organs may be briefly described as follows: 



Nervous System — The central nervous system is slightly more 

 advanced than at the stage of development described by Marshall 

 in the newly hatched tadpole of the frog. The forebrain extends 

 anteriorly between the olfactory pits. The end brain is begin- 

 ning to be divided anterior to the olfactory pit into two hemi- 

 spheres. In the groove between these is a vascular plexus. The 

 pineal body extends forward above the undivided portion of the 

 forebrain. It is composed of cords of cells interlaced with capil- 

 laries. The optic stalks are patent near the neural canal but 

 toward the optic cups are narrow and are apparently partly filled 

 by newly formed fibers. The infundibulum is large, its lateral 

 walls are thick. The roof of the midbrain has begun to thicken. 

 The floor of the fourth ventricle on each side is very thick. The 



