IV. i INITIAL STRUCTURE OF THE GERM 175 



only the ventral lip; it develops mesoderm but undergoes no 

 further differentiation, and eventually drops off. Should the 

 constriction be delayed until after the dorsal lip has appeared 

 both halves may form an embryo, but the ventral embryo is 

 usually imperfect. These differences are attributed by Spemann 

 to differences in the time or place of constriction. 



When a transverse constriction is made after the appearance 

 of the medullary plate the anterior half develops as a whole, 

 with complete nervous system and optic vesicles; the posterior 

 forms a medullary plate, but no folds, and dies. After the 

 appearance of the medullary folds, however, the anterior and 

 posterior halves produced by transverse constriction develop as 

 halves, although a case is described where each had a pair of 

 auditory vesicles. 



When the sagittal plane coincides with the first furrow, con- 

 striction in the two-celled stage gives rise to double-headed 

 monsters ; if the constriction is slight the most anterior organs 

 only are involved (duplicitas anterior), the cerebral hemispheres, 

 epiphysis, hypophysis, and paraphysis being doubled ; there may 

 be two complete pairs of eyes, or the median eyes may be more 

 or less fused (Diprosopus triophthalmus). Tighter constriction 

 brings about a reduplication of the chorda, auditory vesicles, and 

 fore limbs (Dicephalus tetrabrachius). Similar effects are 

 obtained by constriction in the blastopore stage, but not later ; 

 halves separated in the median plane when the medullary folds 

 have arisen die as halves. 



We shall now briefly consider another experiment by which 

 the independence of one another of the parts is demonstrated. 

 Schaper removed the brain, eyes, and, probably, the auditory 

 vesicles from newly hatched tadpoles of Rana esculenta. The 

 wound healed up. The mouth moved to the anterior end and 

 the suckers up the sides of the tadpoles, which lived for nearly 

 a week, in the course of which they grew 2 mm. They then 

 showed signs of weakness and were preserved. It was found 

 that the mouth had opened, that labial cartilages, pterygo- 

 palatine bar, jaw-muscles, gill-bars, gills, heart and blood- 

 vessels, trigeminus and vagus ganglia, oesophagus, pronephros 

 and glomus, and dorsal muscles had all been differentiated. 



