18 
The Embryology of Patella. 
shaped and compressed ones, wbich have on]y recently freed them- 
selves from the mouth of the closing gastrula. In the midst of 
the anterior endoderm cells, wbich are tbe lirst to sbow more 
regularity in size and a more symmetrical arrangement, appears 
a longitudinal opening widest anteriorly and gradually decreasing 
in widtb towards tbe opposite extremity. As tbis slit libe opening 
increases in length, tbe cells at its posterior extremity, wbich 
hitherto have sbown very irregulär and compressed outlines, also 
become cuboidal in sbape and arrange tbemselves in a single row 
around tbis cavity. Tbis sac, or tbe mesenteron, now becomes more 
elongated, its anterior and lateral walls being composed of 
slightly wedge-sbaped cells, tbose at tbe anterior end being 
larger than tbose at tbe opposite extremity. Tbe ventral wall is 
composed of a layer of very much smaller cells, while at first 
the posterior dorsal wall is very imperfect owing to tbe want of 
space caused by the enormous development of tbe sbell gland. 
Tbe large endoderm cells, which just before tbe closure of the 
blastopore entirely filled tbe Segmentation cavity, bave become so re- 
duced in size tbrougb tbe demands made upon tbem by tbe active 
embryo tliat a large space now exists between tbe walls of the 
mesenteron and the ectoderm, wbich constituted tbe body cavity. 
Tbe furtber development of tbe mesenteron eonsists in tbe 
contraction of the elongated sac just described into a smaller and 
spberical one wbich assumes a more posterior position on tbe 
dorsal side of tbe embryo; PI. V, Fig. 58. We have already seen 
bow at the end of tbe preceding section tbe Y-sbaped furrow at 
tbe bottom of wbich was the blastopore became transformed into 
tbe oesopbagus. As tbe blastopore was gradually moved towards 
tbe ventral side tbe few ectoderm cells PI. III, Fig. 41, wbich 
intervened on tbat side between tbe blastopore and the velum, 
became subject to a pressure wbich finally bent tbem inwards 
and upwards , where tbey formed tbe anterior wall of tbe 
apex of tbe V sbaped furrow, and wben the blastopore closes 
tbey form tbe anterior wall of tbe oesopbagus. As tbe oesopbagus 
increases in length a marked difference is seen in the construction 
of its anterior and posterior wall, tbe former being composed of 
small cuboidal cells, while the latter is tbick and made up of 
larger and mucb longer cells. Tbe oesopbagus at first grows 
directly forwards so that tbe thin anterior wall comes to lie 
against tbe velum. As it increases in length the extremity becomes 
bent inward towards tbe centre of the embryo, where it comes 
( 166 ) 
