26 DESCRIPTION OF THE FIGURES. 



this area the cells (trophoblastic) of the outer sac are modi- 

 fied so as to assist in taking up nourishment— the uterine 

 milk — from the uterus, and in fixing the embryo during its 

 uterine development. The allantois {all.) never reaches 

 the outer sac. It is vascular, and serves only as a breathing 

 organ. After Osborn and Selenka. 



Fig. 3. A semi-diagrammatic representation of a four-weeks' (2S 

 days) horse embryo and its foetal appendages. The embryo, 

 which is curved so that the tail lies under the head, 

 measures nearly three-eighths of an inch in length. The 

 limbs are represented by lobes entirely composed of cells, 

 i.e., the rudiments of the limb skeleton have not yet 

 appeared. Behind the head are three arches and three 

 clefts, but the clefts do not appear to open, as in fishes, into 

 the pharynx. The amnion {am.) surrounds the embryo, 

 and two stalks proceed from the under surface. The 

 stalk proceeding to the left side connects the embryo with 

 the yolk sac {y.s.), the stalk passing to the right contains 

 the vessels of the allantois. The allantois {all.) is already 

 in contact with the embryonic sac (d), and with the 

 amnion, and it has many vessels (v l ) in its wall. The 

 yolk sac is vascular (v.), as far as the circular blood-vessel 

 (s.t.), and crowded with granules which have entered 

 by the absorbing area (a, b, c). The cells of the outer 

 tunic at t.g. (on a level with the growing point of 

 the allantois) have undergone considerable elongation, while 

 the cells in the area {a, b, r) have given rise to a number 

 of irregular ridges and processes. 



Fig. 4. Represents a five-weeks' (35 days) embryo. Note that the 

 absorbing area {a, b, c) is smaller than in the 28 days' 

 embryo, while the capacity of the allantois is greater. 

 Delicate villi (c.v.) now project from the allantois towards 

 the yolk sac, and the girdle {t.g.) now consists of distinct 

 nearly parallel ridges, with furrows between — these ridges, 

 as well as the ridges of the ring (an.) around the absorbing 

 area, have been exaggerated in the drawings. 



Fig. 5. The 42 days' embryo and its appendages. Note especially 

 that the girdle {t.g.) lies near the absorbing area {a, b, c), 

 and that the allantois is now a relatively large sac in con- 

 tact with the greater extent of the oixter tunic, and sur- 

 rounding the greater part of the yolk sac. 



Fig. 6. The seven-weeks' (49 days) horse embryo. Contrast this 

 with the other embryos, more especially with the five and 



