222 STUDY OF PIG EMBRYOS. 



out first forward, then makes an acute but rounded angle, and extends outward 

 through the umbiUcal opening. It may, therefore, be said to consist of two limbs, 

 one within the body of the embryo joining the cloaca, and the other passing out 

 through the umbilical opening. The limb arising from the cloaca is completely 

 united with the body-wall, and is, of course, upon the side toward the coelom 

 covered in by mesothelium. The lining of the allantoic cavity is an epithelium, 

 and is a portion of the entoderm. Along the second limb of the allantois the 

 mesothelium on the side toward the cavity of the umbilicus forms a series of 

 clumsy projections, Vil, the mesothelial villi of the allantois. They are smallest 

 toward the embryo and increase in size distally. With higher power one can 

 see that the mesothelium of the villi is very thin and the mesenchyma in their 

 interior of quite loose texture. In later stages the mesothelium grows, the 

 mesenchyma in large part disappears, and the villi then seem hardly more than 

 small bags of mesothelium with but little contents, save some coagulum. They 

 continue to enlarge until the embryo is 17 or 1 8 mm. long, after which they begin 

 to abort. In these older stages the villi extend far into the abdomen and are 

 packed in between the abdominal viscera, presenting curious appearances in 

 section. As the tail of the embryo is bent to one side, it offers us a section of 

 a portion of the spinal cord, Sp. c, and at its tip a glimpse of three primitive seg- 

 ments, Seg. 



Frontal Section through the Mid-brain and Fore-brain. — Comparison with 

 figure 99 (pig, 10 mm.) will make it clear that in a frontal series we shall obtain 

 a few sections of the head which include only mid-brain and fore-brain and show 

 no other special cephalic structures. The mid-brain, M. B, is somewhat rounded 

 in form and passes over into the fore-brain, which is quite long and which already 

 shows traces of its subdivision into two parts, the diencephalon, Dien, which lies 

 nearest to the mid-brain, and the prosencephalon. Pros, which constitutes the 

 terminal portion of the brain and which produces the lateral expansions which 

 are to form the cerebral hemispheres. The expanding prosencephalon is sepa- 

 rated by a constriction from the diencephalon, which in its turn is similarly sepa- 

 rated from the mid-brain. The diencephalon and prosencephalon together rep- 

 resent the fore-brain. They are subdivisions of the primary first cerebral vesi- 

 cles. It is important to note that they do not correspond to complete subdivis 

 ions, and have not the same morphological value as the three primary vesicles. 

 The histological development is much less advanced than in the pig of 12 mm. 

 The ectoderm is very thin, consisting for the most part of a single layer of cells, 

 but here and there the formation of a second layer is seen to be beginning. The 

 mesoderm is very simple in character and almost uniform in appearance, but 

 there is a distinct difference between the mesenchyma around the brain and that 



