456 K. M. PACE. 



divided to form the oral ectoderm. The cells of the oral 

 ectoderm are all equal in size^ though they are larger than 

 those of the aboral ectoderm. A study of sections shows that 

 the ring-like appearance seen at this stage^ when viewing the 

 egg from the aboral surface^ is due simply to this difference in 

 size between the cells of the oral and aboral surfaces causing 

 the former to project out beyond the latter (PI. 24, figs. 

 58-60). In later stages this appearance is enhanced by the 

 development of the aboral groove or mantle cavity just above 

 the line of junction of the two sets of ectoderm cells (PI. 25, 

 fig, 61). Barrois (2) was misled by this deceptive appear- 

 ance, and, not having checked his observations on the living 

 egg by the examination of sections, published figures (2, 

 pi, xii, fig, 6), purporting to represent the larva at this stage, 

 showing a prominent equatorial ring of large cells, while the 

 remaining cells are represented as being of the same size both 

 above and below this supposed ring. 



The Mesendoderm, — Owing to the rapid growth of the 

 oral ectoderm, the four central oral cells eventually become 

 surrounded and enclosed in the segmentation cavity, thus 

 forming, together with the four cells originally segmented 

 off, eight mesendoderm cells in all. These eight cells divide 

 repeatedly, and give rise to a solid mass of tissue, which in 

 time comes to fill the segmentation cavity. PI. 24, figs. 57-60, 

 represent stages in this process. The asymmetrical position of 

 the mesendoderm cells in PL 24, fig. 58, seems somewhat 

 peculiar, and might well be considered to be abnormal, but 

 the arrangement shown in this figure has been met with in 

 all the sections examined, and must, therefore, be regarded 

 as quite normal. 



The Blastopore and Gastrulation , — The oral ecto- 

 derm remains for some time open at the point of enclosui'e of 

 the mesendoderm cells, and this opening may be regarded as 

 the blastopore (PI, 24, figs. 58, 59, Bl). All trace of the 

 blastopore vanishes in later stages (PI. 24, fig. 60). It is 

 impossible to say whether the enclosure of the second set of 

 mesendoderm cells be due to the pressure of the surrounding 



