ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 371 



degree of development readied by the hybrid fertilized ova varied greatly. 

 Fertilization was sometimes effected without any result. In other cases the 

 ova reached the stage of segmentation, and that to a varying extent. A 

 third degree was exhibited by those which survived till the closure of the 

 blastopore. When this was so the preceding segmentation was perfectly 

 regular. Many larvae die soon after liberation, a few survive. 



(2) In hybridization between Bana fusca m. and M. arvalis f., and 

 between Bufo variabilis m. and B. cinereus f., the larvae survived meta- 

 morphosis ; in the latter the hybridization is readily successful, in the 

 former only in a small minority of cases. 



(3) In amphibians hybrid fertilization occurs most readily when the 

 reproductive elements of both sexes are in perfect maturity. The sperm 

 retains its potentiality only a very short time in relation to the ova of 

 some species, for a very long time in relation to those of others. These 

 conclusions are compared with those of Pfliiger and Hertwig. 



(4) The result of hybrid fertilization between B. fusca m. and B. arvalis f. 

 depends greatly on the concentration of the sperm-fluid. The more dilute, 

 the fewer sperms are there to overcome the difficulties of effecting entrance. 

 Probably, too, the sperms have lost some of their energy. The obverse 

 statement that the more concentrated the sperm-fluid, the more spermatozoa 

 should enter the ovum, was beautifully verified by direct observation. Born 

 has proved that the presence of more than one sperm-nucleus within the 

 ovum is usually at the greatest hazard of normal development. All the 

 more marked irregularities in hybridization are referable to polyspermy ; 

 when regular and simple segmentation and development have occurred, it 

 may be conversely inferred that only one spermatozoon has entered. As in 

 normal fertilization, the ova have the power of refusing sperms after one 

 has entered. In most cases, after a certain stage is passed, the divergent 

 tendencies of ovum and sperm seem no longer organically compatible, and 

 a paralysis of development ensues. Life and continued development are 

 coincident. 



(5) It is difficult to determine whether cases of successful development 

 after hybrid fertilization occur normally. Bufo variabilis m. and Bufo 

 cinereus f. are often found copulating, but the result is only known to be 

 successful in the above artificial experiments. 



The research closes with some notes on the manifold obstacles to 

 successful hybridization, on hybridization in general, and on the suggestive- 

 ness of such inquiries as to the pathology of reproduction in relation to the 

 normal processes. 



Origin of Periblast in Teleosteans.* — Dr. J. H. List reports the 

 results of some observations made in 1884 on the formation of the periblast 

 in Crenilabrus tinea, C. quinquemaculata, and C. jpavo. The first part of 

 the paper is occupied with an historical review of the relative researches of 

 Agassiz and Whitman, Kupffer, Wenckebach and others. 



Ten hours after fertilization the blastodisc is seen as a cap on the yolk, 

 with its margin still more than thirty degrees from the equator. Along 

 the whole margin cells stretch out towards the yolk and are constricted off. 

 After separation from the margin these cells arranged themselves in con- 

 centric rows. They remain separated by interspaces, and the cells of one 

 row correspond to the interspaces of the next. Certain irregularities must 

 however be allowed. The cells are all oval, and lie with their long axis 

 parallel to the blastodisc margin. Between them there are abundant fat 

 globules thickest close to the margin of the blastoderm. Dr. List regards 



* Biol. Centralbl., vii. (1887) pp. 81-8. 



