2 WILLIAM A HILTON. 



iH-ntation cavity may in this way normally become filled in, 



m at pri-un unable t> say, but I have found some specimens 



every Xl '-"" t(ir l ' K ' P ast ^ cw >"*-' ars m ^'hich the segmentation 



ra\ity was totally obliterated. As the cells divide, the small 



ones of the animal pole gradually extend 

 about the larger yolk cells and, at an 

 early stage, one may recognize at the 

 point of junction of the two masses and 

 in from the surface, some of the small 

 cells which later go to form the meso- 

 blast. These correspond to those recog- 

 nized by Morgan, King, Ruffini and 

 others. 1 At a stage when the small cells 

 begin to migrate over the larger, there 

 are about three layers of these with 

 small yolk granules at the animal pole. 



This number later becomes reduced to 

 FIG. 2. Portion of a late . 



morula stage of D. fusca. two and then to one, similar to the sin- 



gle cell layer found by Lange in Megalo- 

 batrachns;- this outer layer or ectoblast 



becomes more extensive and may be found forming a simple 

 definite covering over a large part of the egg. The other small 

 (vIN, main- if not all of which contain small yolk granules, have 

 l>y this time migrated considerably, but not so extensively as 

 the ectoblastic. Their exact limits are hard to determine. At 

 about the time the ectoblastic covering at the animal pole has 

 become reduced to one cell, the first indication of gastrulation 

 \\.i^ tound. Fig. 3 is a section of a rather early stage. 



As mentioned at an earlier time, the external appearance of 

 gastrula stages do not differ from those of other amphibia to 

 .m\ greal derive, but some details of development seem peculiar. 

 I no\\ h.i\c several series of earlier stages than have been available 

 and in the earliest of these, 1 find the gastrula mouth 



I. II. Mi'.'atl, "Tin- <lrvrl.|iiin-iit <! tl.< I 1'Vg." 1897. H. D. KillK. 



Che Gastrulation ol tin- LVM i r>n<~ lent iy.inu^n ." .\ i. \iit., \'\. XXXVI.. 1902. 



A. Kiiltini, "Coiurilmto alia mil' i-.-i-n/a ili-lla ( >nt..u' " \mlil>i .mmi rd 



li," Anh. It. Di. Aunt. Di. I-'.inb., K>n-. 



\>. !' I..HIKI-. Jr., "Die Kciiiil>l.i[ti-iliil.lun <!<- .\li-i;<il<>l>,iir<n ln<* inn. \inius 

 Aunt. II- Bd . , II. 3, [907. 



