710 SUMMARY OF CURRENT RESEARCHES RELATING TO 



explanation of the phenomenon described " ; tlio first segmentation 

 nucleus is here, as in all sexual cells, formed by the fusion of the nucleus 

 of the ovum with the sperm-nucleus, and the fusion of the two cells 

 observed at a later stage is something additional to the ordinary impreg- 

 nation. They urge a number of facts in extenuation. They propose to 

 call this additional body the conjugating cell, and at present only apply 

 to it the epithet of enigmatical. 



Hertwig's 'Human and Vertebrate Embryology.'* — In the earlier 

 portion of Dr. O. Hertwig's recently published text-book of vertebrate 

 embryolotry, the sexual elements, the maturation of the egg, fertilization 

 and cleavage, the development of the germinal layers, the blood and 

 connective tissue and egg-envelopes of reptiles, birds, and mammals are 

 described, and the formation of the organs from the epiblast, hypoblast, 

 mesoblast, and mesenchyma. The genesis of the organs from the primary 

 layers is admirably illustrated with sjiccial reference to its bearings on 

 the anatomy of the adult human body, while enough data from com- 

 parative embryology are laid under contribution to give the reader a fair 

 knowledge of the wide application of the principles laid down. It is 

 believed that this little work will be found of great value to the medical 

 student in understanding many questions in pathology, physiology, the 

 structure of the brain and the mechanism of the nervous system. 



B. Histology. f 



Cells and Tissues. | — Prof. F. Leydig has published another sugges- 

 tive essay, which is based on the study of the cells and tissues of Argulua. 

 In dealing with cells he treats first of spongioplasm and hyaloplasm ; 

 it may always be considered an advance in knowledge to be able to break 

 up into structures jiarts of an organism which have been hitherto sup- 

 posed to be of one and the same nature. This is now the case with the 

 cell. In 1876 the author pointed out that there might be (a) concentric 

 striation of protoplasm, as in the ganglionic spheres of Insects and 

 Annelids ; or (f3) striped differentiation, which might be longitudinal, 

 transverse, or radial ; or (y) there may be plexiform differentiation of 

 the protoplasm, as in the cell-nuclei and blood-corpuscles of Triton. 



It is now generally recognized that there are in the cell-substance 

 two substances ; one of these forms a kind of network, and has been 

 called the substantia opaca, the other lies in the interspaces and is soft 

 and clear ; it is the s. hyalina. With these the newer terms of spongio- 

 plasm and hyaloplasm are synonymous. Leydig has also shown that 

 these two parts play a definite role in the conversion of the cells into 

 tissues, and this has been confirmed by Eabl and by Sedgwick. Argulus 

 is well adapted for the kind of investigations the author wished to 

 undertake ; for not only the eggs, but also the large cells which belong 

 to the fat-body, and, especially, the unicellular glands show the plexiform 

 and radiate arrangement of the spongioplasm. The space around the 

 nucleus was distinctly observed. In the large cells of the fat-body it 

 was possible to see numerous nucleoli without any inclosing membrane. 



* Hertwig, O., 'Lehrbuch dor Entwickelungsfjeschichte des Mensclien u. der 

 "Wirl elthicre,' 8vo, Jena, 1887-8, viii. and 507 pp. (figs.). Cf. Amer. Naturalist, xxii. 

 (ISj-S) pp. 179-82. 



t This stction is limited to papers relating to Cells and Fibres. 



X Zool. Anzeig., xi. (1888) pp. 251-9, 274-80, 309-15, 328-33. 



