ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 653 



Experimental Studies in Development of Eye and Nasal Cavity.* 

 B. T. Bell has performed a number of delicate experiments on frog 

 embryos, such as removing half the anterior brain and optic vesicle, 

 turning the optic vesicle round, making the normally inner pole lie 

 outwards, implantation of the optic vesicle of another embryo, and 

 such like. Some of the conclusions deduced from the results which 

 followed are that a small eye may develop in place of one removed ; the 

 retina may certainly be regenerated in very young embryos after removal 

 of its entire anlage ; a typical lens (with lens fibres and epithelium) may 

 be formed (1) from the pigment layer of the retina ; (2) from the brain 

 tissue of another embryo ; (3) from the ectoderm dorsal to the mid-brain 

 region ; (4) from the cord of ectodermal cells forming the primordium 

 of the nasal organ. From analogous experiments the evidence indicates, 

 though it does not definitely prove, that the nasal primordium develops 

 independently of both brain and pharynx, and that its connection with 

 these organs is brought about later by stimuli arising after the various 

 structures have come into relation with each other. 



Development of Thyroid in Bdellostoma stouti.j — C. R. Stockard 

 finds that the thyroid arises as a long trough-like anlage extending over 

 a relatively long gut area, a form probably related to the number of the 

 gills and the extent of area occupied by them. Later, the trough 

 becomes a more or less continuous chain of cell-groups, as if the body 

 of the trough-like evagination, after pinching away from the pharynx 

 floor, had begun to break down or disintegrate into small groups of 

 cells. The nuclei of all the cells in the thyroid tissue are much larger 

 and stain more deeply than those of the mesenchyme cells among which 

 they lie. In an embryo just hatched the thyroid is seen to be in 

 almost the adult condition. It consists of diffusely scattered alveoli 

 below the pharynx and above the median branchial artery. The 

 manner in which the groups of cells produce the alveoli is strikingly 

 analogous to the way in which the morula of a holoblastic egg passes 

 into the hollow blastula stage. There is nothing in the development of 

 the thyroid in Bdellostoma to suggest a paired origin for this organ. 



Primitive Occipital Vertebrae.:}: — R. Froriep discusses the question 

 of the homology of the occipital " Urwirbel " of the Amniota and of the 

 Selachii, the problem as to the correspondence of the cranio-vertebral 

 boundaries in the different vertebrate groups. It appears that in 

 Amniota and also in Selachii there is a long embryonal period in which 

 only three occipital somites persist, but these very distinctly. These 

 are the three necessary for the formation of the posterior part of the 

 head in Selachii, and the persistence in highly developed forms, 

 especially the Mammalia, of three occipitoblasts, is held to warrant the 

 assumption that the two sets of structures are the same. The con- 

 clusion drawn is that they still appear in higher Vertebrates, because 

 throughout the whole range of descent they have always been utilised in 

 occipitalisation. 



* Anat. Anzeig., xxix. (1906) pp. 185-94 (2 figs.). 



t Tom. cit., pp. 91-9 (8 figs.). 



J Op. cit., xxvii. (1905) Erganzungsheft, pp. 111-20. 



Dec. 19th. 1906 2 x 



