STUDIES IN MAMMALIAN EMBRYOLOGY. 509 



parallel to the hypoblast^ so that the protochordal plate 

 becomes — at least the central part of it — more than one cell 

 thick (figs. 28, 40, and 41). 



While this process is going on, the gastrula ridge has, how- 

 ever, made its appearance in the posterior part of the em- 

 bryonic shield, and its shape, extension, superficial groove, 

 mode of development and of production of a peripheral sheet 

 of mesoblast correspond to what has so often been described 

 for other mammals. Still I will enter into a few details con- 

 cerning these processes, yet not without once more insisting 

 on the important fact that the formation of a protochordal 

 plate has preceded the appearance of the very first indications 

 of a gastrula ridge (primitive streak). 



2. The Development of the Mesoblast. 



Towards the origin and further development of the middle 

 layer in Sorex vulgaris three distinct sources contribute. 



It is only for a short period that they and the mesoblast 

 which they produce remain distinct. Soon the mesoblast 

 becomes a confluent plate of cells, more than one cell thick, 

 stretching further and further between the two primary layers, 

 and separated in the median axis of the embryo by the noto- 

 chord or by the tissues that give rise to it. There is every 

 reason to suppose that after a short time the process by which 

 mesoblast originates from the three difl'erent sources above 

 mentioned ceases, and that from this period onward the growth 

 of the mesoblast is due to self-division of pre-existent meso- 

 blast-cells. It follows from this that only by examination of 

 certain favorable early stages we can obtain reliable data 

 concerning the actual origin of the mesoblast. 



The three sources above alluded to are — 



(1) The protochordal plate noticed on p. 508. 



(2) The gastrula ridge and its median prolongation forwards, 

 which advances between epi- and hypo-blast, and which I 

 have proposed to designate by the name of the protochordal 

 wedge (Kopffortsatz, auct.). 



(3) An annular zone of hypoblast situated just outside the 



