518 A. A. W. HUBRECHT. 



twOj is more distinctly^ though not as yet entirely, separated 

 from them along this axial line^ the two lateral plates of meso- 

 blast being, however, continuous with each other, both in 

 front of the protochordal plate and behind the embryonic shield. 

 Moreover, the extra-embryonic coelom has become apparent 

 in the stage 42 all along a curved line, just behind the em- 

 bryonic shield (figs. 79, 80, 83, 87, 91, coel.). The cavity is 

 widest and most spacious just behind the gastrula ridge, in the 

 region where the development of the amnion will begin, 

 and where the allantois will make its appearance. In front 

 this coelom is not yet continued into the mesoblast underlying 

 the epiblastic shield. In one of the embryos 42 (see figs. 80 

 and 88) I find, however, two small spaces symmetrically 

 situated in this mesoblast. I have as yet no definite suggestion 

 to make, although presumably these two spaces may be looked 

 upon as the first indications of the pericardial cavity. The 

 further exposition of these phenomena and the participation 

 of the protochordal plate in the formation of the wall of the 

 fore-gut and pharyngeal membrane (primitive Rachenhaut, 

 Carius), as well as in that of the heart, I wish to reserve for 

 a future publication. 



Chap. II. — Theoretical Considerations on the Gastrula- 

 TioN OF the Mammalia. 



In the preceding chapter a detailed description has been 

 given of the mode of development of epiblast, hypoblast, and 

 mesoblast in the shrew. We have seen that the last-named 

 germinal layer has a multiple origin, and that the exact data 

 concerning this latter fact can only be gathered from a study 

 of certain particular developmental stages which are rapidly 

 passed through. After this the mesoblast continues as a 

 separate layer to increase in extension without revealing 

 anytiiing about its primary origin or the multiple foci of its 

 formation. The notochord, the mesoblastic somites, and the 

 lateral plates of mesoblast are the representatives of the middle 

 layer in these later phases; whereas the formative foci of the 



