POLYEAIBRYONIC DEVELOPMENT IN TATUSIA 603 



mal cells are found just beyond the outer margin of the embry- 

 onic ectoderm, in the space lying between the entoderm and the 

 extraembryonic mesoderm, but such cells are undoubtedly given 

 off from the marginal cells of the ectoderm. 



ORIGIN OF THE SECONDARY BUDS 



The formation of the secondary buds immediately follows the 

 establishment of the primary diverticula, and three or four 

 specimens in the collection show the main steps in the process. 

 However, it will be necessary to secure a closer series through 

 this period of development before a detailed account can be 

 given of the origin of the secondary buds. Each primary bud 

 gives rise to two secondary buds, and consequently there are 

 four secondary diverticula. Each secondary bud carries the 

 rudiment or primordium of an embryo. The first step leading 

 to the development of the secondary diverticula consists in the 

 formation of two thickenings in the wall of each primary bud. 

 One of these areas lies at the tip t>f the bud, while the other 

 appears slightly to the left (as viewed from above) of the tip. 

 The secondary buds then arise from these areas as blind diver- 

 ticula, which extend down along the inner surface of the yolk- 

 sac entoderm. In specimen No. 247 the beginning of the second- 

 ary buds can be seen in the left-hand primary bud (fig. 1). At 

 the point marked 5 is seen a slight protrusion which will form 

 secondary bud No. III. 



The secondary buds soon become recognizable in surface views 

 of living specimens, and appear as four blunt processes from the 

 sides of the ectodermal vesicle. Upon the upper surface of each 

 bud an embryonic rudiment appears, in the form of a white, 

 opaque spot. It is somewhat difficult to make out the exact 

 limits of the different parts of the ectodermal structures, owing 

 to the fact that the entoderm (and the chorionic ectoderm, 

 if still present) tend to obscure the view. This difficulty was 

 obviated by making an outline reconstruction from the series 

 of sections of the blastocyst. 



JOURNAL OF MORPHOLOGY, VOL. 24, XO. 4 



