226 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



the atrophy of the gills. At the same time the eyes become 

 evident, and the horny jaws are cast off. The adult form is now 

 rapidly assumed, the tail gradually shrinking, while vegetable 

 food is given up, and exclusively aquatic life abandoned. 



3. Fate of the Germinal Layers. (1) The ectoderm is early 

 differentiated into superficial and nervous layers (Fig. 66). The 

 former gives rise to the horny layer of the epidermis, the deeper 

 part of which is formed from the nervous layer. The epithelium 

 of the mouth-cavity (stomodseum) and posterior part of the 

 cloaca (proctodseum) are also ectodermic, these structures arising 

 as pits of the external surface which ultimately communicate 

 with the mesenteron to form a continuous alimentary canal. 



The neural plate from which the cerebro-spinal axis originates 

 arises as a thickening of the nervous part of the ectoderm. The 

 essential parts of the sense organs are ectodermic. The nasal 

 sacs commence as pits in this layer, while the membranous laby- 

 rinths are originally depressions of the nervous part of it, which 

 are separated off as vesicles and grow into their complicated 

 adult form. The lens of the eye (cf. Fig. 80, I) is formed 

 as a similar vesicle, which becomes solid. An outgrowth from 

 the anterior cerebral vesicle, the optic vesicle, grows towards the 

 embryonic lens, which meets and pushes in its end, forming a 

 double-walled optic cup. The part connecting this with the brain 

 narrows to constitute the optic nerve, while the inner wall of the 

 cup becomes the essential part of the retina, the outer part its 

 pigment layer. The rest of the eye is mesodermic. 



(2) The endoderm gives rise to the epithelium of the mesen- 

 teron and its outgrowths (Fig. 67, at) i.e., lungs, liver, and 

 pancreas. The visceral clefts are also formed as pouches of the 

 mesenteron. Four of them open as above described to the 

 exterior. In front of these is a pouch which never opens on the 

 surface and becomes the tympanic cavity. From the endoderm 

 the first rudiment of the endoskeleton is formed. This is a 

 cellular rod, the notochord, which underlies the nervous system as 

 far forwards as the pituitary body. Remains of it are found in 

 the centra of the adult vertebrae. 



(3) The mesoderm at first forms a sheet on each side of the 

 notochord below the ectoderm, and also extends into the front of 

 the head. Each sheet splits into an outer somatic layer, and an 

 inner splanchnic layer, the two remaining united dorsally. The 



