STUDIES IN MAMMALIAN EMBRYOLOGY. 519 



composed of cytoblast aud plasmodiblastj which are iu cou- 

 tinuous growth and in rapid increase. 



Of the mother only the blood and the blood-corpuscles pene- 

 trate into the numerous and intricate lacunae of this region. 

 Both the external cap of maternal tissue and the internal core 

 of trophoblastic tissue (with allantoidean villi embedded in it) 

 out of which the placentary region is composed, show the traces 

 of their respective growth in numerous karyokinetic figures. 

 In the stage of fig. 11 these are yet numerous in the maternal 

 crypt region ; in that of fig. 13 they are already rare or absent, 

 and the increase of this layer may be said to have ceased. On 

 the contrary, the number of karyokinetic figures in the cyto- 

 blast is in this stage very striking. Of direct nuclear division, 

 without karyokinetic intermediate stages, examples are found 

 in the plasmodiblast. 



The glands, as far as they persist in the placentary region 

 (sometimes with unexpected local dilatations), are not invaded 

 by trophoblastic knobs or villi ; their openings towards the 

 uterine lumen are closed by the trophoblast, and they play no 

 part in the fixation of the blastocyst, or in the facilitation of 

 the intercourse between the maternal and the embryonic cir- 

 culation. In the latest stages of pregnancy all vestiges of the 

 glands have disappeared in the region between the villi. 



And so we see that there is in the placentary region a 

 gradual substitution of maternal tissue by embryonic tissues, 

 corresponding in extent to what is indicated in figs. 10 — 15 

 by the extension of the red and the black divisions. 



Always with this restriction, that in the region which is in- 

 dicated by black lines maternal blood penetrates into the 

 trophoblastic tissue that fills up all the white space between 

 the black lines. 



Thus the maternal blood is transported from the maternal 

 arteries into channels that are wholly built up of embryonic 

 material, returning back along similar spaces to the superficial 

 (red) covering of the placental region aud to the maternal 

 veins. 



The phenomena of growth and development of the tropho- 



