148 DEVELOPMENT OF THE TADPOLE. 



In our present sections it is easy to distinguish, not only the 

 cerebral hemisi)heres and the medulla oblongata, but also the 

 mid-brain, the optic, and olfactory lobes, as well as in section 2 

 what I believe to be the pineal gland. The whole of the brain 

 and spinal cord is bounded by a single layer of apparently epi- 

 blastic cells placed end to end, so as to form a delicate membrane 

 around it. The fore-brain, also, is at this period well marked oft 

 from the mid-brain by a slight depression and a hollow space 

 within the substance of the brain itself (Fig. 2,/;/.) It is here, 

 just above this depression, that a small oval body is situated, ter- 

 minating in front immediately over the hollow or ventricle above 

 mentioned, but extending backwards slightly over the mid-brain. 

 This oval body is in all probability the pineal gland. If this be 

 so, there is, however, no trace of the eye-structure recently found 

 in the pineal gland of some reptilia, unless, indeed, the very con- 

 siderable depression itself, which exists between the epidermis and 

 limiting membrane of the brain at this very spot, and which is 

 well seen in another section, of the same animal, not given in 

 these drawings, can be taken in connection with the small detached 

 mass of nerve matter as evidence of the existence of an unde- 

 veloped eye. 



The membrane enveloping both brain and spinal cord at this 

 stage would appear to be the commencement of the pia mater. 

 All the organs are, however, surrounded by a similar, though in 

 some cases more delicate membrane. I'hus, the abdomen can 

 be distinctly seen to be bounded by three layers, the innermost of 

 which is bent round in the neighbourhood of the heart to form a 

 large closed chamber, corresponding to the pleuro-peritoneum of 

 the adult, the whole of the already-formed organs being on the 

 one side, and the undifferentiated yelk-mass on the other of this 

 membrane. From the lower side of the spinal cord, at intervals 

 along its length, small processes of nerve-matter can be seen to be 

 extending through the membranous wall; and beneath and around 

 these again cartilage can be perceived to be forming. The spinal 

 cord is at this period perfectly double throughout its wh6le length, 

 and rests upon the notochord, an elastic fibrous sheath, about 

 which more will be said hereafter, binding the two structures toge- 

 ther, although in some of my sections, from which the ligament 



