42 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of axial hypoblast cells, there is no reasDn for doubting the homology of 

 the notochord in these two animals. The mesoblast of the head is not 

 split, and no cavity is formed there. The anus is formed in the middle 

 of the primitive streak. The venous system is very slightly developed, 

 and the blood-corpuscles appear to be formed directly from stellate meso- 

 blast cells. 



Influence of the vertical position on the development of the Eggs 

 of the Chick.* — M. C. Dareste finds that when the obtuse pole of the egg 

 is upwards the evolution of the chick is normal, but that this is not the rule 

 when the opposite pole is the uj^per one. Without oifering a comj)lete 

 explanation of these facts, he points out that the " cicatricula " of the egg 

 is in different conditions. "Whatever be the position of the egg the yellow 

 is always uppermost, and the cicatricula in the highest part of the yellow. 

 When the narrow pole is the higher the cicatricula is in contact with the 

 shell, from which at one time it is only separated by the vitelline mem- 

 brane. When the obtuse pole is su2:)crior, the cicatricula is in contact with 

 the lower wall of the air-chamber, which has a flexible wall. 



Nucleus in Frog's Ovum.f — Dr. G. Thin describes conditions of the 

 nucleus in the ova of Bana temporaria between the stages of division 

 into four segments and that of the appearance of the morula condition ; 

 sections were made from eggs hardened in bichromate of potash and stained 

 with picrocarmine ; these methods were not such as to enable the nuclear 

 network to be satisfactorily made out. 



In the first stage observed — or that of a " tablet-nucleus " — an unformed 

 substance was found infiltrating the yolk in certain parts of the segments. 

 Then came the diffuse granular nucleus in which minute yolk tablets were 

 found in the carmine-stained nuclear area ; in the homogeneous nucleus 

 the nuclear substance stains homogeneously in carmine, has distinct 

 boundaries, and no yolk tablets or pigment. The fourth stage is that of 

 the shrunk nucleus, in which a cresceut-shaped shrivelled homogeneous 

 substance represents the nucleus. Fifthly, simple holes were found which 

 appear to correspond to the position of nuclei. The author gives a short 

 account of the division of the nucleus, of the pigment, and of the pigment 

 in relation to the segment. Pigment has no causal relation in the nuclear 

 changes. 



Embryology of Teleostei.| — Herr H. F. Wenckebach reports the result 

 of some studies on the development of Teleostean embryos, and especially 

 of Belone. 



1. The periblast nuclei. — According to Hoff'mann the first segmentation 

 in Teleostei is parallel to the axis of the ovum, and the upper nucleus 

 divides to form the blastoderm, while the lower divides into the free 

 nuclei of the subjacent protoplasmic layer— the periblast. According to 

 Agassiz and Whitman, however, these last nuclei arise from the marginal 

 cells of the blastoderm. Wenckebach's results go to show that the free 

 periblast nuclei always originate from the blastoderm, either (1) from the 

 marginal cells, or (2) from cells which fall from the lower surface of the 

 blastoderm on to the floor of the segmentation cavity, there fusing with 

 the periblast. The author notes the various relative researches of Ryder, 

 Brook, Cunningham, &c., as to the origin and history of the periblast. In 

 relation to the parablast theory of His, Wenckebach notes that as far as the 



' Comptes Rendus, ciii. (1886) pp. 09G-7. 



t Report Brit. Assoc. Adv. Sci. for 1885 (1886) pp. 1069-71. 



X Arch. f. Mikr. Anat., xxviii. (1886) pp. 225-51 (2 pis.). 



