FIRST MONTH 81 



7 nini. by .V.~> mm., and is entirely covered by villi. 1 The embryonic rudiment projects 

 from the chorion in an oblique direction, and is attached by a broad mesodermic 

 stalk. The yolk-sac is larger than in the earlier stage, being 1'083 mm. in diameter ; 

 the embryonic shield is oval, has indications of a primitive streak, and is some- 

 what concave, the embryonic ectoderm being directly continuous with the flattened 

 ectoderm of the small closed amnionic sac. The connecting stalk contains a 

 short diverticulum of the yolk-sac, the rudiment of the allantois. 



By the end of the second week the chorionic vesicle measures about 8*5 mm. by 

 6'5 mm., and is entirely covered by villi in all the specimens described. In Spec's 



ectoderm 



mesoderm .. ;\<. chorion 



CUMnOn 



yolk-sac 



connecting stalk 

 blood-islands 



embryonic plate 



/? 



rct<n/>'n>i -- ,/ SBGLr"" l v-~ 'chcrion 



mesoderm - 



yolk-sac ....... """%. 



vYv \ ' connecting st-alk 



"^ :; cClliiJ>' allantois 



B 

 FIG. 110 A. EMBRYO OF 0'4 MM. (LETTERED vH) BELONGING TO SECOND WEEK, SHOWING ITS 



POSITION AND ATTACHMENT TO INNER ASPECT OF CHORION. 



FIG. 110 B. THE SAME IN MESIAL LONGITUDINAL SECTION. (After Graf v. Spee, from Kollmann.) 



am, amnion. 



embryo Gle (fig. Ill) the embryonic shield measures 1*54 mm. The embryo is 

 separated by a slight constriction from the yolk-sac, and shows a short primitive 

 groove, a large neurenteric canal, and a shallow neural groove. The yolk-sac is 

 covered by projections caused by the development of blood-islands, and distinct 

 vessels are formed on its lower segment. The connecting stalk has narrowed down 

 relatively, and, compared with the embryo v H, it would appear that in the laying 

 down of the embryonic axis in front of the primitive streak a change in the relative 

 position of parts has taken place, the yolk-sac coming to lie below the shield and 



1 The ovum described by Reichert and one described by Mall of about the same age, or rather older, 

 had merely a circle of villi round the equator, the poles being bare. 



VOL. I. G 



