THbJ DEVELOPMENT (»F LEPIDUSIRKN PARADOXA. 419 



undili'erentiated. From the first the epiblast is thicker in 

 the region overlying- the archenteron, the cells of which it is 

 composed being here somewhat columnar. The fate of this 

 thickened region, which is destined to give rise to by far the 

 most complicated product of the ectoderm, the central nervous 

 system, may conveniently be left out of consideration until a 

 little later. 



In regard to the general ectoderm there is little change 

 to chronicle for a considerable period. In an embryo of 

 Stage 25 it is still two-layered, the bounding surfaces of the 

 two layers being smooth and parallel. Prominent yolk 

 granules are still present in the cells, and the outer layer has 

 formed on its surface a fine but distinct cuticle. When the 

 tail begins to form the ectoderm at the tip of this thickens, 

 its cells assuming a cuboidal form, but remains two-layered. 



The general ectoderm retains its two-layered condition 

 for some time. Increase in thickness by division of the 

 lower layer cells begins at a period varying from about 

 Stage 32 to about Stage 35. 



In various young* Lepidosirens, which have been fixed in 

 strong Flemming's solution, I have been able to make out that 

 certain of the ectodermal colls are provided with tail-like 

 processes of cell substance, closely resembling the tails so 

 characteristic of the coelenterate epithelial cells (PI. 25, fig. 1). 

 The tails of the ectoderuial cells in Lepidosiren are very 

 difficult to observe, showing up distinctly only in well-fixed 

 material in which the cells are slightly separated from one 

 another. They run along the inner surface of the epidermis, 

 forming a kind of plexus-like layer. Into this layer pass 

 also processes from the underlying mesenchyme cells, so that 

 it forms an organic connection between ectoderm and mesen- 

 chyme. 



The glandular structures of the fully formed skin of 

 Lepidosiren (PL 25, fig. 2) are one of its most charac- 

 teristic features. The tall unicellular mucus glands, which, 

 in the adult, form a palisade-like arrangement through the 

 whole thickness of the skin, begin to appear about Stage 35 



