286 MR. a. BROOK ON THE 



Kupifer's vesicle has atropTiied tlie anus seems to be formed 

 pretty nearly at the same point ; but I have not been able to make 

 out clearly the relationship between KupfFer's vesicle and the 

 mesenteron. 



General advance. — At tlie time of closure of the blastopore 

 the brain was already marked out into its three regions, and a 

 lumen had developed in them. Shortly afterwards a lumen is 

 seen extending through the whole length of the cerebro-spinal 

 cord, and before the heart has begun to pulsate the hind brain 

 has been further differentiated, in that the cerebellum has been 

 fully separated off from the medulla oblongata. The nasal pits 

 are now large and well defined. The inner lens of the auditory 

 capsule has increased in size, become hollow, and simultaneously 

 with the pulsation of the heart the -otoliths make their ap- 

 pearance. The lenses have become fully separated from the 

 optic bulbs. The pigment-spots have assumed a stellate form. 

 The first rudiments of lobes to form pectoral fins may be found 

 just before the heart begins to pulsate. Generally also the tail 

 has commenced its growth, and the first folding of the epiblast 

 to form the caudal fin is an accompanying phenomenon. 



Sect. 4. From the Fulsation of the Heart to the time of Hatching. 



Liver and Pancreas. — As the mesenteron increases in size a 

 lumen arises throughout its course and extends quite into the head. 

 At the beginning of the sixth day of development a ventral swelling 

 makes its appearance in that part of the mesenteron lying be- 

 tween the rudimentary pectoral fins — the first rudiment of the 

 liver. This becomes more marked as the tail curves round the 

 yolk, and about the end of the seventh or beginning of the 

 eighth day, the liver presents a lobulated character. Before this 

 time, however, another organ, the pancreas (fig. 23), makes its 

 appearance on the dorsal side of the intestine, and slightly poste- 

 rior to the origin of the liver. It appears soon after the liver 

 arises, and towards the end of the sixth day, as a pocket pushed 

 out from the mesenteron. The mesenteron, up to period of out- 

 growth of the liver and pancreas, was straight, but increasing size 

 of these two organs, as well as of the mesenteron itself, causes 

 the canal to take an S-shaped form, and this becomes still more 

 involved as development proceeds. 



^he proctodeum seems to arise rather late in development. I 



