ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 31 



of the notochord is derived from the mesoblast ; the spinal ganglia are 

 formed from the peripheral parts of the medullary plate. 



In Bana there are no paired rudiments of the mesoblast, and no 

 chorda-endoblast as described by 0. Hertwig; the ccelom theory does 

 not, therefore, apply to the Anura. 



Development of Germinal Layers, Notochord, and Mid-gut in 

 Cyprinoids.* — Prof. W. Eeinhard, in face of the numerous contradictory 

 statements as to the early embryological history of Bony Fishes, has 

 made an investigation into the development of Leuciscm erythrophthalmus. 

 Sections of non-fertilized eggs showed that the yolk was covered by a 

 layer of protoplasm which was collected in large quantities on one side. 

 In the early phases of segmentation the nuclei are of the character 

 described by Kowalevsky in Carassius auratus. The author believes 

 that the periblast is formed from the ingrowing cells of the blastodisc ; 

 these ingrowing cells are amoeboid, and possess a power of movement ; 

 they appear to become arranged in such a way as to give the whole 

 periblast the form of an uninterrupted protoplasmic layer with nuclei 

 scattered therein ; these may increase by direct division. 



In no well-preserved specimen was anything like a segmentation 

 cavity observed, and so far the observations of Kowalevsky are con- 

 firmed ; the cavity figured by Wenckebach seems to be an artificial 

 product. The outer layer of the blastodisc which forms the covering 

 layer does not seem to be invaginated, as List asserts. It and the peri- 

 blast completely cover the yolk, and this layer persists for a long time. 



In later stages of development the periblast makes its way between 

 the higher-lying cells, and reaches the covering layer ; this can only be 

 explained by supposing that the covering layer forms the true ectoblast, 

 by the thickening of which the nerve-tube is formed, and that the cells 

 which lie above the periblast must be regarded as mesoblast, This last, 

 which forms at first a continuous layer, divides later into two lateral 

 masses. An aggregation of some of its cells gives rise at one point to 

 the notochord, which developes from behind forwards. 



The mid-gut appears to be formed thus ; the boundary between the 

 mesoblastic cells and the periblast is, at j5rst, horizontal ; some of the cells 

 of the mesoblast from either side make their way into the yolk, and also 

 press upon the periblast ; in this manner they give rise to a cavity filled 

 by periblast. The cells more to the periphery of this space elongate, 

 and take on the form of the epithelium of the developed mid-gut. This 

 tract does not arise in the form of a solid cord. It closes from behind 

 forwards. The hind-gut is developed earlier. The last signs of the 

 periblast disappear when they are taken up by the development of 

 blood-vessels. 



Origin of Species-t— Prof. G. H. T. Eimer's recent work on the 

 Origin of Species is in part an elaboration and application of results 

 previously reached by the author in his observations on the variation of 

 the wall lizard. J The full title of the present work, of which only the 

 first instalment is yet published, is suggestive as to its contents — " The 



* Zool. Anzeig., xi. (1888) pp. 648-55. 



t ' Die Entstehung der Arten auf Grund von Vererben erworbener Eigenschaften, 

 nach den Gesetzen organischen Wachsens. Ein Beitrag zur einheitlichen Auffassung 

 der Lebewelt,' i. Th., 8vo, Jena, 1888, 461 pp. (6 figs.). 



t ' Ueber das Varieren der Mauereidechse,' Berlin, 1881, 281 pp. (3 pis.). 



