F(ETAL NUTRITION: THE PLACENTA 447 



absorbed and carbonic dioxide excreted are the same, weight 

 for weight, as in the fetal rabbit (Bohr)/ 



Insectivora. — The importance ascribed to the placentation 

 in Insectivora has abeady been referred to (see p. 377). The 

 hedgehog, shrew, mole, and Twpaia have been most fully in- 

 vestigated. 



Hedgehog. — In the hedgehog (Erinaceus europceus), the zona 

 pellucida disappears early, before the expansion of the hypo- 

 blast, which, as in Man, forms a closed vesicle. The chronology 

 of embedding is not yet known. In the earhest stage examined 

 by Hubrecht,^ the blastocyst was 0'22 of a miUimetre in diameter. 

 The outer wall was several layers thick all round its circum- 

 ference, and spaces were already present in it. At a shghtly 

 later stage, the blastocyst grows rapidly and the epiblast is re- 

 duced to a single layer, with numerous viUiform processes at 

 intervals, except for a thickened knob which represents the 

 future germinal area. Even now the name trophdblast may be 

 given to the single layer of epiblast with its projections, ex- 

 cluding the thickened knob which is formative and gives rise to 

 the embryonic ectoderm and the Uning of the amniotic cavity. 

 The mesoblast, as yet one-layered, which extends between the 

 trophoblast and hypoblast, consists of an attenuated somatic 

 part which forms with the trophoblast the diplo-trophoblast,^ and 

 a splanchnic part which forms blood-vessels and blood. 



The early blastocyst comes to rest, as in the mouse, in an 

 anti-mesometrial furrow of the mucosa. It is not yet deter- 

 mined whether any changes occur previously in the uterus ; but 

 at least, soon after the blastocyst has taken up its position, there 

 is a great cell-prohferation in the stroma of the floor and walls 

 of the furrow, not perivascular as in the rabbit, but sub- 

 epithehal. Along with this decidual formation, the lumina of 

 the glands are closed, and their epitheUum gradually disappears, 

 perhaps by the influence of the trophoblast. The capillaries 

 are distended and new vessels are formed. This distension is at 



' Hubreoht, " The Placentation of Erinaceus europceus," Quar. Jour. 

 Micr. Sci., vol", xxr., 1889. 



^ Hubreoht restricts the term chorion to Tarsiua (a lemur), monkeys, 

 apes, and Man. 



