54 



THE EARLY DEVELOPMENT OF MAMMALS. 



openings grow until the ventral wall of the notochordal canal is entirely lost. We 

 then have the two cavities completely fused, making a single cavity bounded by a 

 continuous layer of cells, the majority of which represents the lining of the yolk- 

 cavity, but the small minority represents the cells of the notochordal canal. The 

 continuous layer of cells is known as the permanent entoderm, and the cavity itself, 

 which is of double origin, is termed the archenteron. At about this time, probably 

 sometimes earlier, sometimes later, according to the species, the blastopore becomes 

 permanently closed and the entodermal cavity no longer has an opening to the 

 exterior. v 



i2Si Atnn 



FIG. 20. GERMINAL AREA OF A GUINEA-PIG AT FIG. 21. LONGITUDINAL SECTION OF THE POSTERIOR 



THIRTEEN DAYS AND TWENTY HOURS, SEEN 

 FROM THE UNDER (ENTODERMAL) SIDE. 

 o. a, Area opaca. a.p, Area pellucida. nch, Noto- 

 chordal canal with several irregular openings 

 through the entoderm. (After Lieberkilhn.) 



END OF A SHEEP EMBRYO OF SIXTEEN DAYS. 

 Amn, Amnion. a.m, Anal membrane (or plate), pr.s, 

 Primitive streak. En, Entoderm. Ach, Archenteron, 

 or entodermal cavity of the embryo. All, Anlage 

 of allantois. mes, Mesoderm. (After R. Bonnet.) 



In a number of vertebrates it has been demonstrated that the blastopore is 

 soon divided into two parts: one anterior, which frequently remains open, and 

 gives rise to the neurenteric canal, and one posterior, which gives rise to the anal 

 opening. When the spinal cord (medullary canal) is developed it extends so far as 

 to include the neurenteric canal and exclude the anus. The neurenteric canal is 

 obliterated during early embryonic life, but so long as it remains open it constitutes 

 a free communication between the archenteron and the medullary tube (spinal 

 cord). The anal opening is early closed by a growth of the surrounding cells, 

 which produces an occluding membrane known as the anal plate (Fig. 21, a.m). 

 The plate includes a layer of ectodermal and of entodermal cells, but apparently 

 no mesoderm. It persists for a long time and undergoes a considerable growth, 

 but ultimately it is perforated to form the permanent anus. 



The cells on the dorsal side of the notochordal canal have a different destina- 

 tion, for they become thickened to make the anlage of the future notochord. It 

 is to this fact that the canal owes its name. 



