28 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



SECTION 4. The Organs of the Embryo outlined in Follicle Cells. 



The structure of the young embryo and the shares which the two 

 sorts of cells take will be understood from the plates, and especially from 

 the horizontal sections in Plates XII, XIII, XIV, XVI and XVII, and 

 from the diagrammatic reconstructions in cuts A, B, C and D. 



The shape of the young embryo makes it difficult to control the 

 position of sections in any plane except the horizontal ; that is, the plane 

 which is parallel to the bottom of the page in Plate XXXV. I have 

 therefore paid especial attention to serial sections in this plane, and have 

 figured a series from young embryos at successive stages of development, 

 in Plates XI, XII, XIII, XIV, XVI and XVII. Longitudinal sections 

 like Plate XXXV, and vertical transverse sections like those in Plates 

 XVIII, XXII and XLV, are much more intelligible if they are perfectly 

 symmetrical and exactly at right angles to the horizontal plane, but as 

 the slightest deviation makes them hard to interpret, I have not been 

 able to obtain, from young embryos, any which are exact enough to be 

 useful for illustration. They have been valuable to me in the interpre- 

 tation of the horizontal sections, but an attempt to describe them would 

 complicate the description so much that I have not drawn them, but 

 have, instead, reconstructed from them, and from the horizontal sections 

 which are figured, the series of diagrams of vertical transverse sections 

 which is shown in cuts A, B, C and D. 



We left the follicle at the stage, Plate XI, Fig. 1, at which its cavity 

 is entirely filled up by the visceral layer, 8, which is in direct contact 

 with the somatic layer, 7, although the inner ends of the somatic cells are 

 sharply defined. These two layers soon become separated again by a 

 space, Fig. 3, 15, which persists from this time as the permanent body 

 cavity of the embryo. It is colored purple in all the figures, and is 

 marked 15. It is probably the original cavity of the follicle, opened a 

 second time by the growth of surrounding parts, and at the stage of Fig. 

 3, and for a long time in the later history of the embryo, it is bounded on 

 all sides by follicle cells. A diagram, constructed from a series of trans- 

 verse sections of an embryo, like Fig. 3, is shown in cut A on page 29. 

 In this, as in the plates, y is the body cavity of the chain-salpa, B is the 

 epithelial capsule, 15 is the body cavity of the embryo, 9 and H' are the 

 blastomeres, and 7 and 8 the somatic and visceral layers of follicle cells. 



On each side of the middle line the somatic layer is invaginated to 

 form a pit which opens into the space between the embryo and the epithe- 



