PLATK LXXII. 



Kit;, i',7. Similar preparation at the twelfth day. The blastoderm has increased considerably in width, ami 

 shows at n a marginal thickening. Opposite the thinner central portion, rf, the blastoderm is separated from 

 the yolk, c, by a hollow space, the cleavage cavity, b. It is still, however, connected with the yolk by 

 colunins of cells, the sub-geniiinal processes. 



$. 

 P 



Fir,. 169-172. Sections of the ei.' of Imf.i cinercus, intended to show the relations hetwevn the d 

 cavity ami Huscoiii's cavity, eventually the visceral cavity (after Strieker). R. The dorsal a-; 



B. The ventral a*i>ect, I. I'.aer's elea\agc cavity. X. In 169 and 170, Rusconi's cleft; in 170, Hnsc 



(A;il,riii.-!/xh<i/,l>-). I'. l>ome of the clvi\ a.w cavity, consist in- ,,f cl..inent.s in an advaiirci! .stage nf ele:n. 

 representinif the original upper jxJc of the egjf. r. Original lower ]M)le of the egg, si in 17! 



and 172, Ecker's yolk plug. t. Kln.ients ,,f the mar-ins of the cleavaw cavity (cnitral >"lk HUM of l:,-i- 

 chert). They are larjrer, tliat is, less aihancetl in clca\a.^T, than the elements in the iinic of tin- 

 cavity or of Ru.scoiii'-s i-a\i(y. In nr/, they are makinir tlieir way aloli- the inni-r sm fact.- of the CUM-I- of the 

 cleavage cavity towards tin- upper ]H,le. They answer to the formative ele.nenis of the trout's tgg. I: 

 cleft advam-e.s l'teen these ehmenty, so that in tjt, where the cleft has In-come a ea\it>, Ufc 

 from the cleavage cavity l>y a layer of formative elements, s. In 17.', owini: To the alteration in its centre of 

 gravity, the egg has changed its ]Kksition, the wliitc ]Mi!e Ix-inj; How nearly uppermost, p. 152. 



