SUMMARY OF CURRENT RESEARCHES. 631 



critical moment when various stimuli iuay induce parthenogonetic de- 

 velopment. In the case of Asterias these stimuli sometimes occur 

 " naturally." In most of the experiments with Asterias the extrusion 

 of the second polar body was inhibited by the artificial conditions. 



Origin of Germ-cells in Chick Embryo.* — Prof. M. Nussbaum brings 

 forward some evidence, which he does not regard as quite sufficient, to 

 show that the primitive ova arise from " sex-cells " which on the second 

 day of incubation are seen to lie apart in the splanchnopleure of the 

 posterior region of the body. They divide mitotically and are shunted 

 towards the middle, and seem to form the germinal primordium. 



Structure of Embryonic Bone-marrow. f— J. Aug. Hammar points 

 out that it is commonly stated that the bone-marrow of the human fcetus 

 is of the red type, but that this statement is only true in the later stages. 

 At the time of its first appearance, the bone-marrow has the characters 

 of a richly vascular connective-tissue, leucocytes being absent. At the 

 beginning of the fourth month of intra-uterine life, a lymphoid infiltra- 

 tion begins, and continues as the ossification of the bone proceeds, until 

 typical red-marrow is formed. This red-marrow is but a stage in the 

 development of yellow-marrow, which has thus two antecedent stages. 



Brain of Erinaceus4 — Gosta Grbnberg has investigated the develop- 

 ment of the brain in the hedgehog, as a primitive type of Eutherian 

 Mammals. His results show the existence of certain detailed homo- 

 logies with structures found in the lower Vertebrates, not hitherto 

 described ; but generally speaking they only confirm the views currently 

 held as to brain development. A distinct superior commissure was 

 discovered in front of the epiphysis, and the processus infundibuli was 

 found to be homologous with a part of the saccus vasculosus of Fishes 

 and Amphibians. 



Mammalian Spermatogenesis^ — H. Schoenfeld begins a contem- 

 plated series of studies on this subject with an account of the period 

 of growth of the genital cells in the bull. He finds that during this 

 period, the spermatocytes of the first order undergo a series of nuclear 

 changes resulting in the formation of twelve annular chromatic segments. 

 These annuli are formed by the fragmentation of a longitudinally split 

 chromatic rod, and the subsequent fusion of the extremities of the 

 double segments thus produced. The spireme results from the juxta- 

 2>osition of tetrads, formed by the division of chromatic granules. The 

 chromatic granules in their turn arise from the union of chromatic 

 microsomata, produced by the disintegration of nuclear chromoplasts. 

 The linin supporting the spireme is not the same as that which exists 

 in the nuclei of the spermatocytes at the time of their formation ; this 

 disappears entirely. Until the moment when the spireme undergoes 

 longitudinal division, the nuclear chromatin is in the state of synapsis, 

 that is to say, it is under the attractive influence of the central corpuscles, 

 the attraction being especially marked at the moment when the tetrads 

 are in existence. 



* Yerli. Anat. Ges., 15 Vers., in Anat. Anzeig., xix. Erg.-Hft. pp. 38-10. 



t Anat. Auzeig., xix. (1901) pp. 507-70 (3 rigs.). 



t Zool. Jahrb., xv. (1901) pp. 2C1-3S1 (6 pis. and IS figs.). 



§ Arch. Biol., xviii. (1901) pp. 1-72 (2 pis.). 



2 U 2 



