FUSION OF EPIBLASTIO LATEES IN RABBIT AND FROG. 167 



that may be, they seem to form that which may, as a whole, 

 be regarded as a reticulum, which is comparable to the myelo- 

 spongium of His. 



The outlines between the cells of the nervous layer are quite 

 imperceptible in unstained specimens of this stage. The 

 nuclei of the nervous layer are only seen with great difficulty. 



The boundaries of the epidermic layer cells can in most 

 cases be readily perceived, owing to the deeper pigmentation 

 of the cells of this layer. The processes {PR.) are sharply and 

 clearly defined, and are heavily loaded with pigment. 



The bodies of the epidermic epiblast cells remain as yet 

 lining the interior of the neural canal, although there is cer- 

 tainly a tendency for the nuclei in some of them to move more 

 inwards. Also there seems to be a tendency for the epidermic 

 cells to become pressed apart by the nervous cells. This is 

 more marked at this stage in the spinal cord than in the brain, 

 as may be seen in fig. 4. 



I think we may take these appearances as a conclusive proof 

 of the occurrence of an intimate fusion of the two epiblastic 

 layers in the frog. 



Observe the condition of the auditory vesicle in fig. 3. Here 

 the walls of the vesicle are composed entirely of the light- 

 coloured elements. There is no trace of the dark, deeply- 

 pigmented strands such as are to be seen in the central nervous 

 system. This is a further piece of evidence that the dark 

 strands in fig. 3 are derived from the epidermic layer. For, as 

 is well known, in the frog the nervous layer alone gives rise 

 to the auditory vesicle — so it is a most significant fact that the 

 dark strands should be entirely absent in this case. 



After this stage (3 — 5 mm.) it is extremely difficult to trace 

 the fate of the epidermic and nervous cells respectively. Up 

 to now, the fact of the greater pigmentation of the former has 

 rendered the inquiry easy. 



The origin and meaning of the pigmentation is obscure, but 

 its presence in the frog seems to be due to two causes, separate 

 at any rate chronologically. 



Firstly. Pigment is present in the unfertilised ovum as a 



