Abnormal Development of Toad Ova 35 



Musculature — The muscles of the head are well differentiated. 

 In places the spinal myotomes are fairly normal; in places the 

 cells composing them are much scattered. 



Skin — The epithelium shows irregular projecting processes in 

 many places. 



Larva No. 6, Experiment II, Plate V 



External Fortn — The long axis of this embryo is nearly straight. 

 The head appears much crumpled. The mouth is a large cavity 

 bounded by irregular folds of tissue. There is a small semi- 

 circular sucker back of the oral opening. 



Central Nervous System — The brain is anteriorly much dilated 

 (section a) and extends to the anterior extremity of the head. 

 The neural wall is very thin and the central cavity is filled with 

 degenerated cells. The dorsal wall of the forebrain in places is 

 fused to the ectoderm (section h). There are no visible traces of 

 a pineal body. The ectoderm is irregularly thickened where it 

 comes in contact with the brain and in some places is very thick. 

 In one place a long column of ectoderm cells extends in between 

 the ectoderm and the brain (right side of section b). The dorsal 

 wall of the brain on each side in the region of the eyes is greatly 

 thickened. The midbrain is most irregular inform and gives rise 

 to several vesicular processes of uncertain nature. The walls of 

 the midbrain are thin and the central canal is large. The medulla 

 is most abnormal in form (sections c and d). The dorsal part of 

 the fourth ventricle is curiously dilated. The spinal cord is more 

 normal in form than the brain although it also is much deformed 

 in places. 



Organs of Special Sense — The only trace of nasal epithelium 

 is a collection of cells between the dilated pharynx and the ecto- 

 derm in the anterior part of the head shown on the right side of 

 section a. The eyes are very abnormal. The optic stalk is 

 dilated so as to make a direct opening from the neural canal to the 

 back of the sensory layer of the retina. There is no trace of 

 specific differentiation in this layer (section b). Where the optic 

 stalk approaches nearest to the ectoderm a lens has been differen- 

 tiated. This is still in contact with the ectoderm and consists of 



