ZOOLOGV AND BOTANY, MICROSCOPY, ETC. 289' 



the germinal nuclei. Only the row of germinal nuclei has the power of 

 division. 



There are three well-defined periods of growth : («) the period of 

 cell-multiplication' (second to eighth day) ; (b) the period of re-adjnst- 

 ment (eighth to tenth day) ; (c) the period of final differentiation (tenth 

 day to the end of the incubation). Up to the end of the first period the 

 retina grows from within outward by the deposition of an additional 

 row of nuclei with each successive generation. After this, karyokinetic 

 figures are found only at the margins. 



Differentiation begins at the centre of the retinal cup, and spreads 

 towards the growing margins. The nearer the margin the younger are 

 the stages seen. The sranglion-cell layer consists at first of three rows 

 of nuclei ; these fall into line in the direction of the internal limiting 

 membrane, so as eventually to form but one layer. 



In the inner nuclear layer, differentiation into horizontal cells, fibres 

 of Miiller, and bipolar cells takes place jyari passu with the formation of 

 these nuclei. This layer consists at first of about fourteen generations 

 of nuclei. AVitli the exception of the horizontal cells, each successive 

 generation fails to attain quite the size of the one preceding. The 

 number of rows of nuclei decreases from fourteen to eight by a closing, 

 up of the ranks in the direction of the external limiting membrane. 



Up to the end of the period of re-adjustment, the nuclei of the inner 

 nuclear layer are elliptical in outline, with the long axis at right angles 

 4.0 the external limiting membrane. Later they become circular. The 

 reticular layers are cytoplasmic in both origin and structure. 



The pigment layer is a direct continuation of the retina, and, like 

 it, begins as a syncytium. The nuclei, at first in two rows, are sooa 

 arranged in one, with the stretching out of the surface area. Active 

 growth is restricted to the margins of the pigment layers, and continues 

 as long as the eye increases in size. The nuclei remain of the same size 

 as those in the early undifferentiated condition. Pigment granules first 

 form on the side of the layer away from the retina and in the protoplasm 

 l)etween the nuclei. They are never normally found outside the cyto- 

 plasm of the pigment-cell, and there is no evidence that pigment is a 

 food-substance. The numerous large blood-vessels in the choroid coat 

 next the pigment layer may furnish the nutritive material for the develop- 

 ment of the retina. 



The outer nuclear layer represents the last two generations from the 

 division of the germinal nuclei. The rod nuclei are the youngest in the 

 retina. They are more numerous than the cone nuclei, and division 

 among them does not wholly cease until the rods of their immediate 

 neighbours have attained a high degree of development. The rods 

 and cones have their origin in undifferentiated cytoplasm, and there is no 

 evidence that any part of the rod or cone is of nuclear origin. The nuclei 

 of the rods and cones retain their early position wholly within the 

 external limiting membrane, and are not protruded beyond it, as has 

 been recorded for other animals. In the development of the retina there 

 is no fixed time for the appearance of the different elements. 



Origin of Lung in Ceratodus.* — Greil points out that between 

 the seventh and last of the branchial outgrowths of the gullet and the 



* Anat Auzeig., xxix. (1906) Erganzungsheft, pp. 115-31. 



