ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 439 



The dorsal aorta arises from cells or protoplasmic masses derived 

 from the sclerotome ; its lumen is derived from the fusion of originally 

 separate vacuoles in these masses ; the endocardium appears to be 

 mesoblastic in origin ; the blood-corpuscles appear suddenly, and it is 

 suggested that they are mesenchyme cells set free by an epidemic of 

 mitosis. 



The chondrocranium is amphibian-like in early stages. The neural 

 tube arises by overarching of the medullary folds. Both infundibulum 

 and optic rudiments are clearly recognisable while the medullary groove 

 is still widely open throughout. As in Lepidosiren and other forms, the 

 brain is, during the earlier part of its development, divided into two, 

 not three regions— the primitive forebraiu and the rhombencephalon. 

 The pineal outgrowth is single, and without any eye-like structure. 

 In the adult the cerebellum becomes highly developed, and forms 

 anteriorly a valvula cerebelli, while posteriorly it projects back in a 

 quite similar manner into the fourth ventricle. The material forming 

 the side walls of the thalamencephalon does not become pushed out to 

 form cerebral hemispheres, but is accommodated partly by the great 

 increase in the length of the thalamencephalon, partly by its becoming 

 invaginated into the interior of the third ventricle. The two olfactory 

 rudiments are apparently connected by an ectodermal thickening across 

 the middle line in early stages ; the cavity of the olfactory organ is a 

 secondary excavation in the originally solid rudiment. 



On the whole, the general phenomena of development in Polypterus 

 show frequent striking resemblances with what occur in Dipnoans and 

 in the lower Amphibia. In the investigator's opinion these resemblances 

 are sufficient by themselves to indicate the probability that the Teleo- 

 stomes, the Dipnoans, and the Amphibians, have arisen in phylogeny 

 from a common stem, which would in turn probably have diverged from 

 the ancestral Selachian stock. The ancestors of the Amniota ; probably 

 diverged either about one or about several points from the region 

 of the stem common to Dipnoi and Amphibia. 



The external gills develop in Polypterus exactly as they do in 

 Lepidosiren and Protopterus, and in the more primitive Amphibia 

 (Urodela and Gymnophiona), i.e. each one arises as an outgrowth from 

 the outer side of the visceral arch (in this case hyoidean), composed of 

 mesenchymatous core and an ectodermal covering. They appear before 

 the perforation of the gill-clefts, and are probably organs of great 

 antiquity. The respiratory epithelium of the gill-clefts has arisen by a 

 spreading inwards from the ectodermal respiratory epithelium of the 

 external gills. 



The author returns to his theory that paired limbs are honio- 

 dynamous with external gills in which the potential motor function 

 has been accentuated. 



Budgett showed that the condition of the fin-skeleton in the larva 

 of Polypterus indicates its close relationship to the type of uniserial fin- 

 skeleton in sharks ; Graham Kerr re-states the hypothesis that both can 

 be referred back to a primitive biserial archipterygium like that of 

 Ceratodus. 



In the evolution of the head there has been a varying amount of 

 displacement in an antero-posterior direction of the relative positions 



