I. INTRODUCTION. 



Desirous of making a study of the so-called inversion of the germinal layers in 

 rodents, and wishing for purposes of comparison to use a family of the order other 

 than those usually studied, I determined several years ago to work upon the sub- 

 order Sciuromorphi, for the reason that it had been almost entirely neglected. There- 

 upon I began collecting Spermophilus as the most easily obtained representative. 

 In the first collections there were many stages lacking; but I soon discovered that 

 Spermophilus was not in accord with the descriptions of inversion then published, 

 and that it had developmental features peculiar to itself, which were far from being 

 easily understood owing to want of material. For several seasons I collected specimens 

 in large numbers, believing that in this way I could secure a series with few stages 

 lacking. This has proved to be the case, and I now have material from which I am 

 working out in detail the complete development of this animal. In a preliminary 

 notice (Lee, :02), read before the American Morphological Society, Dec. 29, 1901, 

 I briefly outlined the subject-matter of this paper. 



i. Material. — The small striped gopher, Spermophilus tridecemlineatus Mitch., 

 is found abundantly throughout Minnesota living in burrows in pasture-lands and 

 grain-fields. While a timid animal, it has a fatal curiosity which is of the utmost 

 service to the collector, for, after retreating, it soon reappears at the mouth of its 

 burrow to be caught in the snare or shot with the rifle. At the imitation of its shrill 

 whistle the gopher will often stop when running to its burrow and stand erect to dis- 

 cover the source of the whistle; or, if in its burrow, its reappearance may be hastened 

 by this means. In the breeding season the whistle acts as a challenge to the males, 

 and one can often bring them within a few yards. .In this way needless shots are 

 avoided. The gopher has the habit of sitting at the margin of its burrow so that it 

 can tumble in at the least alarm. It has astonishing powers of vitality, for even 

 when mortally wounded it often reaches its burrow. One should endeavor to shoot 

 them through the head or back, as otherwise a large percentage is lost. It breeds 

 in the spring, the males making their appearance as soon as they cease hibernating, 

 usually between April 10 and 25. The period of gestation, as near as I can determine, 

 is about one month. I have not been able to breed it in captivity, consequently 

 I do not know the exact duration of this period. At any given time the majority of 



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