550 LOCY. [Vol. XI. 



Amblystoma the optic vesicles are well developed in very 

 early stages. In Torpedo ocellata they show fairly well, and in 

 the chick they may be readily made out while the groove is 

 widely open. 



In Squalus acanthias the optic vesicles are the first rudi- 

 ments of sense-organs to appear. Counting out the gastricular 

 cavity (and the neural segments) they are the first organs of 

 any kind to be established. Strangely enough, their early his- 

 tory in these animals has been entirely overlooked. Balfour's 

 statement that " The eye does not present in its early develop- 

 ment any very especial features of interest," seems to have 

 withdrawn attention from that organ in the elasmobranch 

 fishes. 



In order to fix clearly the time when the optic vesicles first 

 appear in Squalus, we must glance over the early steps in the 

 formation of the head-plate. PI. XXVI, Figs, i, 2, 3, rep- 

 resent three stages that immediately precede the formation of 

 the eyes. In Fig. i the embryo is well established ; it is a 

 stage slightly older than Balfour's stage B. The front part of 

 the embryo is not broader than the rest, and the curve of the 

 anterior margin is uniform and unbroken. In Fig. 2 we note 

 two changes, viz., that the front end is broader than the part 

 just behind it, which is apparently constricted, and the anterior 

 margin is drawn out into a rounded median cusp. The head- 

 end continues to expand laterally (Figs. 3 and 4) until we may 

 (with some appropriateness) speak of it distinctively as a head- 

 plate. In the meantime the trunk region has grown longer, 

 and is relatively narrower, so that the entire axial embryo has 

 a characteristic appearance of a narrow body terminated by a 

 rounded plate-like head. When this stage has been reached the 

 first satisfactory traces of the eyes become visible ; when first 

 formed they present the appearance that might be produced by 

 pressing down lightly upon a plastic surface with two rounded 

 dies. They are circular areas slightly concave upwards, and 

 occupy a position far forwards on the cephalic plate. The 

 early stage of these vesicles may easily escape observation, as 

 they become evident only when the light strikes them properly, 

 and when they are in a favorable position for the observer. 



