570 THE EMBRYOLOGY OF THE FROG 



while many hundreds of volumes and articles have been written on it 

 from many angles, there are nevertheless hundreds of interesting points 

 in that animal's development which are unknown. In fact, one writer 

 says that the gaps that confront one in the study of the frog assume 

 "remarkable proportions" when one thinks of how much work has really 

 been done on this much-studied animal. 



Then, too, as there is no accurate method of obtaining the age of 

 a frog, owing to the remarkable influence temperature and food play in 

 its development, it is often difficult to make clear much that should be 

 made clear to the student. 



Roughly speaking, at the time of hatching, namely, about one or 

 two weeks after fertilization, the larvae of most species are about six 

 or seven millimeters in length. The tadpole is usually about nine or ten 

 millimeters in length at the time of the opening of the mouth, and eleven 

 or twelve millimeters when the limb-buds appear. 



THE NERVOUS SYSTEM 



There are no neuromeres in the brain region of the frog, though 

 otherwise the same brain divisions take place that we have discussed in 

 the chick. 



The tuberculum posterius (Fig. 332, tp) is a thickening opposite the 

 tip of the notochord in the floor of the brain, while a dorsal thickening 

 appears in the roof of the brain obliquely upward and forward from this. 

 A plane passing from the tuberculum posterius in front of the dorsal 

 thickening separates the fore-brain from the mid-brain, while a plane 

 passing from the same tuberculum behind the dorsal thickening sepa- 

 rates the mid-brain from the hind-brain. 



The beginning of the brain divisions is quite similar to the three 

 primary regions mentioned in the chick embryo, and a review of the 

 matter there given will make the divisions and cavities in the frog un- 

 derstandable. 



It will be remembered that the olfactory lobes and the cerebral hem- 

 ispheres form the telencephalon, and that the telencephalon and the 

 "tween-brain" (diencephalon) together form the fore-brain, while the 

 optic lobes and optic chiasma form the main portions of the mesencepha- 

 lon. The cerebellum takes up most of the metencephaloti, while the 

 medulla oblongata forms the myelencephalon. 



THE FORE-BRAIN 



Opposite the neuropore (Fig. 328) the cells of the ectodermal cone 

 scatter, due to the pushing out of the head tissues in advance of the 

 brain. Soon all trace of the neuropore disappears except for a slight in- 

 dentation known as the olfactory recess, and this also disappears a short 

 time later. 



The lamina terminalis (Fig. 282, C) is a thickening just below the 



