MUSEUM OF COMPARATIVE ZOOLOGY. 315 



" Meyen was the first to give this structure the name of placenta, 

 and his account of it is so very clear and precise that it is wonderful 

 it should have been subsequently forgotten or overlooked. He says: 

 ' Wir haben bei ganz jungen Individ uen den Verlauf der Blut-beweg- 

 ung selbst by 200-maliger Vergrosserung beobachten konnen. Der 

 Muttertheil der Placenta hat nur wenig Gefasse, um so rnebr aber der 

 Fotus-theil, in dem sich ein ausserordentliches Convolut von Gefassen 

 befindet, das sich in einem Stamme endigt, der sich in das grosse 

 Bauchgefasse ganz in der Nahe des Herzens ergiesst. Ein unrnit- 

 telbares Uebergehen der Blutgefasse aus dem Muttertheil in den 

 Fotus-theil haben wir nicht sehen konnen. Hat der Fotus die hinlan- 

 gliche Ausbildung im Leibe der Mutter erreicht, so verwiichst das 

 grosse Blut-gefass und die Placenta fiillt ab.' — Page 440." 



Krohn, Huxley, Leuckart, and Vogt subsequently investigated the 

 structure of this organ, and have correctly described its more impor- 

 tant features as they exist when fully formed, but its origin and the 

 nature and structure of the club-shaped core are here described for 

 the first time.* 



The embryo, at the stage last described (Fig. 20), consists of the 

 outer tunic derived from the outer layer of the gastrula, the body 

 cavity, and the branchial sac, the wall of which is formed from a por- 

 tion of the cells of the inner layer, while the remainder form a large 

 mass (14), at what is to be the posterior end of the body. The subse- 

 quent development of these parts, as well as the formation of those 

 which subsequently make their appearance, will now be described. 



The Outer Tunic. — Little need be said of this ; it is not at first 

 covered by the test, which makes its appearance later, apparently by 

 excretion from the surface of the outer tunic ; the latter now covers 

 all of the embryo except the part in contact with the placenta, and 

 is reflected on this to form the outer wall of the outer or foetal 

 chamber, as already described. It is entirely separated, at first, from 

 the branchial sac by the body cavity, but at a later stage an invagi- 

 nation appears at the anterior end (Fig. 22, e), which unites with the 

 anterior end of the branchial sac, and a perforation appears which 

 forms the incurrent opening (e, Figs. 1, 3, 23, 24, 25, 33, 34). The 



* Leuckart says (p. 53), that the residual yolk becomes the placenta, hut we have 

 seen that this becomes invaginated to form the digestive layer of the germ, and that the 

 placenta is not formed until the close of the gastrule stage. 



