360 SUMMARY OF CURRENT RESEARCHES RELATING TO 



endoderm of the blastocyst : the latter gives origin to the tropho- 

 blastic or extra-embryonal ectoderm (tropho-ectoderm). The formative 

 cells, the homologue of the inner cell-mass of the Monodelphia, are 

 always freely exposed at the surface of the blastocyst, and are never, 

 even temporarily, enclosed by the tropho-ectoderm, as appears always to 

 be the case in the Monodelphia. 



The first cleavage in Didelphys aurita tends to be unequal, and is 

 differential, separating the formative from the non-formative. The 

 unsegmented ovum must be potentially polar in its constitution. 

 Perhaps the first cleavage in Monodelphia is similarly differential. 



J. A. T. 



Lead-poisoning of Germ-cells.— L. J. Cole and L. J. Bachhuber 

 {Proc. Soc. Exper. Biol, and Med., 1914, 12, 24-9). Two series of 

 experiments with rabbits point to the conclusion that the offspring 

 of males which have been poisoned by the ingestion of lead acetate 

 into the alimentary canal have a lower vitality, and are distinctly 

 smaller in average size, than the normal offspring of unpoisoned males. 

 Other experiments with fowls indicate that poisoning of the male parent 

 with lead results in offspring of a distinctly lower average vitality. 



J. A. T. 



Seminiferous Tubules of Mammals.— G. M. Curtis {Amer. Journ. 

 Anat, 1918, 24, 339-94, 24 figs.). Observations were made on 

 isolated tubules of adult mouse, adult rabbit, and three-week dog. Adult 

 tubules present, in their course, no blind ends, blind diverticula, or 

 nodular enlargements. They show in their simpler form an arch, both 

 tubule ends being open and having a functional connexion with the 

 rete. But they form linked series of arches, as many as twelve arches 

 occurring linked in one complex. In rabbit and dog lobules are present, 

 composed of an entire tubule surrounded by a connective tissue sheath. 

 These are divided into sublobules, each with a separate sheath, which 

 contains a part or parts of the single tubule. The waves of the semi- 

 niferous tubules were studied in detail. The average wave-length in 

 the mouse is 1*83 cm., in the rabbit, 1 • 4 cm. J. A. T. 



Spermatogenesis in Mouse.— Harry B. Yocum {Univ. California 

 Puhlications in Zoology, 1917, 16, No. 19, 371-80, 1 pi.). In the 

 common house mouse the primary spermatocytes have twenty chromo- 

 somes, which correspond in number, shape, and seemingly in size varia- 

 tion to those in the ovum. In the first division all of the chromo- 

 somes divide. In the secondary spermatocytes some sections show only 

 nineteen chromosomes, while others show nineteen in one plane or focus 

 and one in another. There is one chromosome which does not divide 

 in the second division, but passes in advance toward one pole of the 

 spindle. There is no secondary pairing of the chromosomes such as has 

 been described in horse, pig, guinea-pig, opossum and man. The 

 spermatids are dimorphic, half having nineteen, and half having twenty 

 chromosomes. j. a. X. 



Spermatogenesis of Dog.— Julian Y. 'NLa.-lo^-r {Trans. Amer. Micr. 

 Soc, 1918, 37, 97-110, 2 pis.). The spermatogonia show deeply 



