302 DR. A. MILNES MARSHALL. 



sides of the brain, and takes a course downwards and 

 outwards, at right angles to the longitudinal axis of the 

 head. Histologically it consists of roundish or oval nu- 

 cleated cells, with, as yet, very few nerve fibres, agreeing 

 completely ■with corresponding stages of development of the 

 other cranial nerves. 



Fig. 20 is taken from a section through the same region 

 as fig. 19, but from a dogfish embryo at the commencement 

 of stage o. The magnifying power employed is the same 

 in the two drawings, so that an exact comparison can 

 be made between them. There is still no indication of a 

 division into cerebral hemispheres; the forebrain, as in fig. 

 19, is still undivided. Though the embryo has grown con- 

 siderably the olfactory nerve (i), though somewhat thicker, is 

 no longer in fig. 20 than in fig. 19, a fact of some interest; 

 its point of attachment to the brain has, however, shifted 

 down somewhat towards the ventral side. The most im- 

 portant fact shown by fig. 20 is, however, the existence of 

 the earliest rudiment of an olfactory lobe {ol. v.). This, as 

 may be seen from the figure, is exceedingly small, and might 

 indeed be easily overlooked ; it is a small shallow pit, 

 formed almost <?ntirely at the expense of the inner wall of 

 the forebrain, and situated opposite the root of origin of the 

 olfactory nerve. 



In fig. 21, taken from one of Mr. Balfour's specimens, the 

 same parts are shown at a stage intermediate between stages 

 o and p. The olfactory vesicle {ol. v.) is seen to have grown 

 very rapidly, and is now a conspicuous object. The olfactory 

 nerve (i), on the other hand, has remained almost stationary 

 as far as size is concerned ; it has, however, undergone con- 

 siderable histological change ; the cells composing its proxi- 

 mal part or root of origin are more elongated and fusiform 

 than before, while beyond this part the nerve presents a 

 ganglionic expansion consisting mainly of roundish cells, 

 similar to those which previously constituted tli« whole 

 nerve, and which gives off, distally, bundles of nerve fibres 

 distributed to the^Schneiderian folds of the olfactory mucous 

 membrane. 



The condition of the olfactory nerve and lobe at stage q 

 is shown in fig. 22, also taken from one of the specimens 

 lent me by Mr. Balfour, who has described this stage as 

 follows : — " The lateral ventricles are now separated by a 

 median partition, and a slight external constriction marks the 

 lobes of the two hemispheres ; these, however, are still united 

 by nervous structures for the greater part of their extent. 

 The olfactory lobes are formed of a distinct bulb and stalk, 



