ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 739 



througli a stage resembling the latter, but tbis is less marked in the extreme 

 Jouannetia, where the closure of the anterior mantle aperture by the mantle 

 diaphragm occurs at a very early stage when the anterior shell opening and 

 foot are still present. This also can be interpreted in association with 

 external conditions and the form of the young shell. The memoir, of which 

 the results are outlined above, is obviously conspicuous for its attempt not 

 merely to chronicle, but to rationalize the morphological facts. 



Molluscoida. 



o. Tunicata. 



Normal and Teratological Embryology of Ascidians.* — M. L. Chabry 

 has investigated the normal embryology of Ascidia aspersa, and has also 

 made a number of experiments and observations on artificially produced 

 monsters. 



In its normal segmentation the egg of this species has considerable 

 resemblance to that of Clavellina rissoana, as described by E. van Beneden 

 and Julin, and that of Clavellina sp. studied by Seeliger. Notwithstanding 

 the apparent irregularity of the segmentation, it has been found possible to 

 homologize the early cells with those of animals not closely related, and 

 these points are explained by the aid of a diagram. 



In the formation of the blastodermic layers A. aspersa most closely 

 resembles Phallusia mammillata ; attention is drawn to the segmented con- 

 dition of two lateral mesodermal bands as an indication of metameric seg- 

 mentation ; the author establishes the primitively double character of the 

 eye, otolith, and notochord. 



With regard to the origin of normal monsters, the author shows that 

 most arise from monstrous germs formed by certain, parents ; the following 

 are the chief points in monstrous formations : — There is a deviation of a facet 

 of segmentation. Segmentation is limited to the nucleus, or is retarded, or 

 does not happen ; the cells migrate abnormally, or fuse, or die. The 

 number of monstrous forms is, naturally, immense, and they are so connected 

 one with another as to render a single natural classification quite 

 impossible. 



The experiments made on the production of monsters have an interest 

 to the general physiologist, or to the student of the special facts of embryology 

 and teratology. As to the former, we have evidence that the production 

 of " traumatismes cellulaires " causes the affected cells to change in form, 

 consistency, and appearance in a manner which is as sudden as it is remark- 

 able. They die rapidly, while the cells which have not been affected 

 undergo correlative changes in form and position ; from this the author 

 concludes that each blastomere has a natural form, different from that which 

 it can take in the egg, where it is in contact with the blastomeres. The 

 blastomeres, are, in fact, ovoid, elastic masses, movable on one another ; 

 the actual form of each of them and their reciprocal action are the 

 result of a mechanical equilibrium, due to their attraction, their natural 

 form, and their hardness. Every segmented ovum, whether normal or 

 abnormal, is a system in equilibrium, and it is impossible to alter the 

 position or the form of any one of its parts without the others passing 

 spontaneously and immediately into another state of equilibrium. The 

 reciprocal attraction or cohesion of the blastomeres is also the physical cause 

 of other phenomena ; it explains the continuity of the dorsal cord which 

 persists in most monsters, the integrity of the ectoderm, and so on. 



* Journ. Anat. et Physiol. (Robin) xxiii. (1887) pp. 167-319 (5 pis.). 



