THK DEVKLOPMMNT OV l.Kl'l POSIHEN FAIIADOXA. 421 



Degeneration of the Cement Organ. — The process of 

 atrophy of the cement organ is a comparatively rapid 

 process, taking place about Stage 35. It is illustrated by 

 fig. 3 D. 



In the early stages of degeneration the glandular epi- 

 thelium becomes penetrated by vascular loops, and leucocytes 

 begin to concentrate in its neighbourhood. At a later stage 

 (e. g. Stage 35, fig. 3 d) there are crowds of leucocytes 

 collected about the gland, and it is now seen that the}' are 

 laden with fatty and other granules, the product of their 

 active metabolism. The glandular part of the ectoderm 

 becomes gradually consumed, and the adjoining epidermis 

 becomes shrivelled and has its surface thrown into wrinkles 

 as the o'land cushion diminishes in size. 



Pigment Cells. 



About Stage 35 bianched pigment cells begin to appear in 

 the ectoderm. I believe that these are all mesodermic in 

 origin. In sections from em])ryos of about this stage many 

 examples of pigment-laden chromatophores may be seen in 

 process of migration into the ectoderm. 



The only case, in fact, that I have found of pigment 

 granules being formed to any conspicuous extent in epidermal 

 cells is that of the pigment layer of the retina. 



Changes in Chromatophores caused by Alteration 

 in the Amount of Incident Liadit. 



I have already referred (Part I, p. 320) to the remarkable 

 difference in the appearance of a young Lepidosiren 

 during the day and night. A Lepidosiren of Stage 38, 

 which by day is of a deep rich brownish black, becomes at 

 night-time quite colourless, the change being associated with 

 the withdrnwnl of the dendritic pseudo]io(li:i of the chro- 

 matophores. An inspection of PI. 25, figs. 4 a and 4 u, will 



