DEVELOPMENT OF THE TADPOLE. 209 



contained in the primitive optic chamber." Ultimately the lens 

 assumes a nearly spherical form, being composed of concentrically 

 arranged layers. 



In PI. XX., Fig. 9, we have a peculiar section of the eye, 

 which appears to show one stage in the formation of the retina 

 and its connection with the optic nerve ; but it possesses so many 

 points of resemblance to Lankester's description of the formation 

 of the lens (above quoted), that since reading this I am in grave 

 doubt as to whether it does not refer to the lens and surroundings. 

 Although not well shown in the diagram, the central, suspended 

 oval portion consists of two or three concentric, transparent layers, 

 and as shown is entirely unconnected from every other part except 

 by its point of attachment. 



In the "Philosophical Transactions" for 187 1 may be found a 

 minute and interesting account of the development of the skull of 

 the frog, by Dr. Parker, together with splendid diagrams of various 

 stages in that development. Fig. i, PI. XIX., shows one stage in 

 this development, as seen in a section of March 18th. In this we 

 see the branchial arches on each side {br.), together with some 

 gills, the formation of these having well commenced as apparent 

 outgrowths of the epidermis. The growth of these gills reminds 

 me very closely of the manner of reproduction by budding which 

 occurs in the hydra. A knob first makes its appearance, the 

 epidermis having all the appearance of being stretched and 

 pushed outwards by some inward force, which, continually acting 

 outwards and forwards, converts the knob into a rounded 

 protuberance. 



Two or three other smaller knobs form 013 this (Fig. 2, PI. 

 XX.), and when the original gill has thus increased considerably 

 in length, it buds laterally, forming a rake-like structure, through 

 which the blood courses bathed externally by the water. These 

 gills are perfectly transparent, the course of the blood-globules 

 being distinctly observed. The blood does not at first seem to 

 follow any definite course through the gill-substance, but for a 

 time appears to take the line of least resistance, soon, however, to 

 become imprisoned in definite channels, the globules following 

 each other in the most orderly progress. 



I was once looking at a specimen in which the gills were about 



