124 THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 



Van Durme (1914, p. 118) has gone into the question in detail from the mito- 

 chondrial point of view in birds. He describes the origin of vitelline granules by 

 a direct transformation of mitochondria and by the initial deposition of vitelline 

 substance in a clear vacuole and its subsequent growth. 



Coghill's observations (1915, p. 349) on the relation of mitochondria to yolk 

 are perhaps the most interesting^ since he alone worked with living cells. His 

 investigations point to the conclusion that the change takes place in the reverse 

 direction, that is to say, from yolk to mitochondria. He studied with great care 

 yolk-laden cells of amphibian embryos which he stained vitally with janus green. 

 He observed structures actually originating from the surface of the yolk globules, 

 by a chemical transformation of their substance, which migrated into the rest of 

 the cytoplasm, where they became indistinguishable from the other mitochondria 

 in the cell. Their morphology, their reactions to janus green, and their staining 

 properties when fixed were identical with those of true mitochondria. From this 

 he concluded that the mitochondria arise from the yolk. Without subscribing to 

 this view, it may be said that these results are not so much at variance with those 

 of other workers as they might at first sight appear to be; for many reactions are 

 reversible and there is no reason why this should be an exception to the rule. 

 Then again, the amphibian embryo is an entirely different tissue from the avian 

 or mammalian egg and there is no reason why the processes going on in it should 

 be identical. 



The possible relationship of mitochondria to the formation of neutral fat has 

 been under discussion since the time of Altmann (1889, p. 94), Metzner (1890, p. 

 82), and others; but Dubreuil (19116, p. 264) has given the most detailed as well as 

 the most recent account of it. The relationshij) of mitochondria to droplets of 

 hpoid in nerve-cells I have already discussed (1914o, p. 13). Reference should 

 also be made to the work of Azzi (1914, p. 7). 



OTHER PRODUCTS. 



The problem of the origin of the external segment of the rods and cones of the 

 retina is a very intricate one, but a few words of description will serve to make it 

 a little clearer. It is well known that the intimate structure of the rods and cones 

 is essentially the same, though their form differs. The constitution of the outer 

 segment of each is alike, so far as our present methods reveal it. It is very dense, 

 especially about the periphery, which is bounded by a sort of envelope. It fre- 

 quently exhibits a transverse striation, just as if it were made up of superposed 

 disks. Authorities are not agreed as to whether this occurs in the living condition 

 or whether it is sim{)ly the result of the technique employed (see Mawas, 19106, 

 p. 115). A distinct longitudinal striation has been described by Ranvier in 

 batrachians, but it does not seem to obtain in other forms. The core of this outer 

 segment contains, according to Leboucq (1909, p. 597), an axial filament which 

 is attached to a centrosome in its most proximal part. 



There is also a close resemblance between the structure of the inner segment 

 of the rods and of the cones, but it is somewhat more complicated than the outer 

 segment, is less dense, and possesses more fluid cytoplasm. There is no hint of 



