POLYEMBRYONIC DEVELOPMENT IN TATUSIA 613 



such as we have just considered the vesicle grows rapidly in 

 size, quickly filling up the entire lumen of the uterus. The first 

 noticable change in growth affects the free or distal portion of 

 the wall, and the chorionic vesicle soon becomes bulb shaped 

 (fig. 5). 



The expansion and growth of the wall of the blastocyst grad- 

 ually carries distally the common amniotic vesicle, which by this 

 time has ceased to grow. As a result, the embryonic rudiments 

 are gradually separated from the amniotic vesicle, and their or- 

 ganic connections with the vesicle are drawn out into small 

 tube-like structures, the amniotic connecting canals. These 

 canals lie against the under or inner surface of the entoderm, 

 and each consists of an inner layer of ectoderm, surrounded by 

 a layer of mesoderm. 



In plate 11 are shown five sections, taken at different levels, 

 from a specimen in which the amniotic vesicle was just begin- 

 ning to be drawn away from the embryonic rudiments, through 

 the expansion of the wall. The sections are not quite transverse 

 to the axes of the four embryos. 



The section represented in figure 80 cuts the tip of the chori- 

 onic vesicle and passes through the lower portion of the common 

 amniotic vesicle. The anterior parts of three of the embryos 

 are seen in the section. These are the dorsal, right-lateral, 

 and ventral embryos. The position of the other embryo, or 

 the left-lateral, is indicated by the evagination at the left side 

 of the vesicle. 



In figure 81, which is three sections further down, the same 

 relation with reference to the embryonic rudiments still exists, 

 but the section passes through the extreme lower limit of the 

 amniotic vesicle, and the right-lateral embryo becomes entirely 

 separated from the others. 



In figure 82, which is five sections lower down on the chorionic 

 vesicle than figure 81, the dorsal and right-lateral embryos are 

 both free from any connections with the amniotic vesicle. In 

 section each embryo appears as a section of a tube. The chief 

 interest in this section lies in the condition of the ventral and 

 left-lateral embryonic rudiments. Only the anterior tips of 



