204 DEVELOPMENT OF THE TADPOLE. 



having regard to its minute structure, which bears no indistinct 

 resemblance to the brain at an early stage, as well as to its 

 epiblastic origin, we are forced to the conclusion that its real 

 function is to act as a special sense organ, enabling the animal, 

 before it possesses eyes or ears, to place itself by feeling in com- 

 munication with that small portion of the outer world which forms 

 its limited surroundings. When more specialised organs are pro- 

 duced, these primitive ones atrophy, as they are of no further use. 



As, up to the period to which we have arrived, one of the 

 most prominent structures seen in all the sections has been the 

 notochord, we will trace its history in some detail. In Plate 

 XIX., Fig. 2, of the present number, will be found a diagram of 

 the anterior end of this structure, as it existed in an animal of 

 March 20, enlarged 100 diameters ; but more or less complete 

 indications of it may be found in all the previous plates, with the 

 exception of the first. Reference should be made to these, and 

 especially to Figs. 7 and 8, Plate II., of January, 1888, and also 

 to Plate XIII., of the July part, for various longitudinal sections. 



Our previous observations led us to the conclusion that the 

 whole of the organs of the animal were produced from either the 

 epiblastic, hypoblastic, or mesoblastic layers. In Amp/iioxus, 

 the Ascidians, the Lamprey, and Lepidosteus, it has been 

 demonstrated by several observers that the notochord is derived 

 from the hypoblastic layer. Towards the anterior portion of the 

 embryo — that is, the portion which ultimately forms the head — 

 certain of the hypoblastic cells split into two, so that whereas the 

 hypoblast is formed of a single layer of cells in the greater portion 

 of the embryo, yet at this particular point it consists of two 

 layers, the under one of which becomes the notochord. This 

 double layer of hypoblastic cells extends from the front to the 

 hinder end of the embryo in an axial line, nearer to what will 

 ultimately be the dorsal than the ventral side. The distinct 

 formation of the lower layer of cells into the notochord occurs 

 first towards the front, and gradually extends towards the back. 

 Gotte has thrown grave doubts as to whether this is the mode of 

 formation of the notochord in the case of the frog and other 

 Anura. He states that the notochord is formed, not from the 

 hypoblast at all, but from the mesoblast. His statement, 



