Soe G. CARL HUBER 
ectoderm, from a narrow zone of cells situated in the region of 
the future primitive streak. From this region there is an out- 
wandering of cells which invade the potential cleft between pri- 
mary embryonic ectoderm and entoderm, spreading laterally 
in wing-like sheets. This I would regard as prostomial mesoderm 
in the sense of C. Rabl. The anlage of the mesoderm in the 
albino rat, and the early stages of its lateral extension, with the 
anlage of the primitive streak and groove, falls to the latter 
part of the ninth day after insemination. 
Beginning with the pronuclear stage, found at the end of the 
first day, 8 days are required for the completion of the process 
of segmentation, blastodermic vesicle formation and the forma- 
tion of the primary germ layers—ectoderm, mesoderm, and 
entoderm—in all, 9 days out of a possible 21 to 23 days, the 
normal gestation period of the albino rat. 
CONCLUSIONS 
Early stages of mammalian development may readily be 
obtained from the albino rat (Mus norvegicus albinus). When 
care is exercised, mating may be observed and the age of the 
embryo, reckoned from the time of mating (insemination), 
determined with a fair degree of accuracy. Ovulations occur 
about the time of parturition and again 29 to 30 days post partem. 
This latter period is more favorable for obtaining insemination 
and semination, thus fertilized ova. The process of fertilization 
probably takes place during the latter half of the first day after 
insemination. 
The pronuclear stage, a stage which extends through a period 
of perhaps 12 to 15 hours, in the middle phase, is observed at 
the end of the first day after insemination; the fertilized ova 
having wandered about one-fourth of the length of the oviduct 
by that time. Of the two pronuclei, the female pronucleus is 
slightly the larger. The two pronuclei lie near the center of 
the ovum, are distinctly membraned, and do not fuse prior 
to the formation of the first segmentation spindle. 
The formation of the first segmentation spindle and the first 
segmentation occur during the early part of the second day after 
