1881.] On the Structure and Develojwient of Lepidosteus. 113 



The section on the general development is followed by a series of 

 sections on tlie adult anatomy and development of various organs. 



The Brain. 



The authors give a fuller description of the adult brain than pre- 

 vious anatomists. The new features in this description are (1) that 

 ^he parts identified by previous anatomists as the olfactory lobes, 

 are really parts of the cerebral hemispheres ; the true olfactory lobes 

 being small prominences at the base of the olfactory nerves ; (2) that 

 there is attached to the roof of the thalamencephalon a peculiar vesicle, 

 which has not hitherto been noticed, but which is similar to the vesicle 

 found by Wiedersheim on the roof of the thalamencephalon of 

 Protopterus. They further show that the cerebrum is divided into 

 a posterior portion, with an unpaired ventricle, and an anterior portion 

 in which the ventricle is paired. They consider the presence of a 

 portion of the cerebrum with an unpaired ventricle, to be an indication 

 that this part of the brain retains characters which are only found in 

 the embryonic brain of other groups. They point to the presence of 

 lobi inferiores on the infundibulum, of tori semicirculares in the mid- 

 brain, and of a large cerebellum as indications of an affinity between 

 the brain of Lepidosteus and that of Teleostei. In the embryological 

 section full details are given as to the development of the thalamen- 

 cephalon, the pineal gland, the cerebrum, and the olfactory lobes. 



At the end of the section the characters and affinities of the Ganoid 

 brain are dealt with at some length ; and the authors attempt to show 

 that brains of Ganoids are distinguished (1) by the large size of the 

 thalamencephalon, and (2) by the cerebrum being divided into an 

 unpaired portion behind and a paired portion in front. 



Organs of Special Sense. 



Olfactory Sacs. — An account is given of the development of the 

 olfactory sacs, in which these sacs are shown to originate as invagina- 

 tions of the nervous layer of the epiblast ; the communication between 

 the sacs and the exterior being effected by the rupture or absorption 

 of the superficial epidermic layer of the epiblast. The double opening 

 of these sacs in the adult is described as arising from the division of 

 the primitive single opening. The olfactory nerve arises as an out- 

 growth of the brain prior to the first differentiation of the olfactory 

 bulb as a special lobe of the brain. 



Eye. — In the adult eye a vascular membrane is described bounding 

 the retinal aspect of the vitreous humour. This membrane is supplied 

 by an artery piercing the retina close to the optic nerve, and the veins 

 from it fall into a circular vessel placed at the insertion of the iris. 

 The membrane itself is composed of a hyaline ground substance with 

 numerous nuclei. 



