. Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



independent bulgings outwards of the wall of the primitive forebrain. During the later stages of development 

 the hemispheres grow rapidly though it is not tili much later than the last stage figured that they approach 

 the extraordinary size and complexity (Elliot Smith, 1908) characteristic of the adult. As regards details 

 of brain development it is to be noted that the pineal body is simple. There is no obvious indication of 

 a parapineal body, and there is at no time any trace of eyelike structure. There is a well marked paraphysis 

 closely resembling that of Urodeles and the velum is clearly paired in origin. 



Peripheral nerves. (Graham Kerr, 1904.) 



Leindosiren with its large cell elements is a very favourable object for studying the development of 

 nerve trunks. Already in stage 24 i. e. while the myotome is still in close contact with the spinal cord the 

 motor trunk can be seen as a naked bridge of soft granulär protoplasm continuous at its outer end with 

 the substance of the myotome and at its inner end with that of the spinal cord. 



As development goes on this protoplasmic bridge becomes fibrillated — neurofibrils appearing in 

 its substance — it becomes drawn out in length as the myotome recedes from the spinal cord with the 

 growth of the trunk and heavily yolked masses of mesenchymatous protoplasm aggregate round the nerve 

 and spreading along it form the primary sheath. 



Olfactory organ. (Graham Kerr, 1901 d, 1909.) 



The olfactory organ (in Protopterus) makes its appearance as seen in external view as a rounded 

 dimple on each side of the under surface of the head some little distance in front of the line of junction 

 of the yolky buccal rudiment with the ectoderm. This dimple becomes gradually elongated to form a groove 

 which passes outwards and backwards. 



When the lips begin to grow out the whole of the olfactory grooves become enclosed within the 

 Upper lip, the hinder end of the groove soon becoming hidden as the lower jaw grows forwards. The 

 olfactory groove assumes a dumb-bell shape dilated at either end and reduced to a narrow slit in the inter- 

 vening portion. By the fusion of the edges of the slit-like portion the two dilated ends become separated 

 off as the anterior and posterior nares. 



While the openings of the olfactory organ develop in the manner above described it is to be noted 

 that the cavity in the inferior of the organ takes its origin as a secondarily arising split in the at first solid 

 rudiment derived from a thickening of the deep layer of the ectoderm. In the later stages of development 

 of the olfactory organ a conspicuous feature is formed by a rounded diverticulum of its lateral wall corre- 

 sponding closely with the similar structure occurring in the embryo Urodele and possibly homologous with 

 the organ of Jacobson of amniotic vertebrates. 



Eye. (Graham Kerr, 1901 d.) 



The optic "outgrowth" of the brain is at first solid as is the thalamencephalon at this stage. 

 A cavity develops secondarily in the optic outgrowth and becomes continuous with that which has meanwhile 

 appeared in the thalamencephalon. The lens develops as a solid ingrowth of the deep layer of the ecto- 

 derm in which a cavity soon appears secondarily. There is a wide choroid fissure which however closes 

 very soon, and which is restricted to the optic cup. From the size of the individual elements Lepidosiren 

 IS a very favourable object for studying the development of the visual cells. The main features in the 

 development of the rods have been described and iigured in an earlier paper (1901 d). 



