ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 491 



has been seen to come into contact with the egg; its head, which is at 

 first homogeneous, soon becomes resolved into a small thread formed of 

 chromatic granules which are connected with one another by filaments, 

 just as in the case of Ascaris viegalocephala, as described by Van 

 Boncdcn. The mode of union of the two pronuclei is carefully 

 described. The segmentation-nucleus is comparatively large. The first 

 plane of segmentation passes through one of the meridians of the egg, 

 and the first two blastomeres are exactly equal and similar ; it will be 

 remembered that this is not the case in the Rabbit. Wiien there are 

 eight blastomeres they are all e€[iial, but exhibit a tendency to become 

 arranged in two distinct groups. When there are twelve blastomeres 

 four are larger than the rest. 



Reproduction and Development of Teleostean Fishes.* — Mr. J. T. 

 Cunningham gives an account of his observations on the ova of Teleostean 

 fishes, made at the new Marine Biological Laboratory at Plymouth. The 

 Common Sole was found to spawn in March, April, and May ; the ovum 

 after extrusion is of considerable size, about 1 • 5 mm. in diameter. It 

 is distinguished by having an immense number of oil-globules of very 

 small size ; these are arranged in groups of irregular shape ; another 

 characteristic is that the yolk is not perfectly continuous and homo- 

 geneous, but coextensive with the blastoderm there is a single superficial 

 layer of separate yolk-masses ; this layer extends with the blastoderm, 

 so that when the latter has enveloped the yolk the layer of yolk-segments 

 also envelopes it completely, forming a superficial layer over the whole 

 surface of the yolk. These peculiarities enable the sole's egg to be 

 easily recognized when taken on the open sea in the tow-net. Mullus 

 and Solea are the only genera whose ova have undoubtedly the peripheral 

 layer of yolk-segments. It is interesting to notice that these ova present 

 a condition of the yolk intermediate between that characteristic of non- 

 pelagic ova and that seen in typical pelagic ova. It is possible that the 

 peculiar character of the ovum of Solea indicates that there is no close 

 affinity between this genus and Pleuronecies. 



After describing a number of ova and his experiments with them, 

 Mr. Cunningham propounds a hypothesis concerning oil-globules in 

 pelagic teleostean ova. He finds that whenever the adult has a large 

 quantity of oil in its tissues, the ova possess one or more oil-globules in 

 the yolk. It is probable that the excess of oil in the tissues of the 

 parents extends into the ovum, and during the development of the 

 latter supplies the embryo with an abundance of fat which is necessary 

 to its constitution. The cause of many ova which are provided with oil- 

 globules having a greater specific gravity than those that are without 

 them must be explained by the greater density of their protoplasm 

 and yolk. 



In conclusion, there is a note on the development of the vascular 

 system and coelom in pelagic ova of Teleostei. In a great many the 

 heart, at the time of hatching, consists of a tube which opens posteriorly 

 out of a wide space between the yolk, while the heart itself is surrounded 

 by another cavity separated from the just mentioned space by a thin 

 membrane ; the cavity which communicates with the heart exists, at an 

 earlier stage, as a space between the epiblast of the anterior part of the 

 yolk-sac and the periblast ; traced back it is found to be nothing more 



* Journal Marine Biol. Assoc, i. (1889) pp. 10-54 (6 pis.). 



