108 Cx. 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 



