DEVELOPMENT OF RANA 



191 



the brain it forms the pia mater (vascular) and dura mater 

 (protective). The eye-muscles arise from mesodermal tissue 

 which represents the three first somites of the head. 



The ears arise as a pair of ingrowths from the ectoderm 

 behind the eyes, forming the auditory vesicles. Their 

 connexion with the ectoderm becomes severed and the remains 



Fig. 85. — Transverse sections through the head of embryos of Rana showing 

 the development of the eyes. 



A, early stage, in which the optic vesicles (ov) have been pushed out on 

 each side from the forebrain (fb). B, the outer walls of the optic vesicles 

 have been pushed in, converting them into optic cups (oc) ; the lens (/) 

 arises opposite the mouth of the optic cup from the ectoderm (ec). C, late 

 stage ; the cavity of the optic vesicle has been almost obliterated, the lateral 

 layer of the optic cup is the retina (r) and the median layer is the pigment 

 layer (pi), the stalk attaching the optic cup to the forebrain is the optic 

 nerve (on), the lens has become detached from the ectoderm. 



of the connecting stalk is the ductus endolymphaticus. Each 

 vesicle now forms a closed sac at the side of the hinder part of 

 the brain, and above the tympanic cavity, which develops as 

 an expansion of the hyomandibular visceral pouch (Eustachian 

 tube). From the dorsal portion of each vesicle three shelf- like 

 projections are formed. The centre of each shelf becomes 



