302 
K. W. MACBEIDE. 
blastopore, Avliich lies beneath the cover of the ectodermic 
folds and forms a space at the hinder end of the animal into 
which the neural canal also opens, viz. the neurenteric 
canal. This is shown in fig. He {ne.) two sections further 
back in the series. In fig. 11 cZ Ave see on the mid-dorsal Avail 
of the archenterou a band of cells undergoing mitosis. 
According to Legros and Cerfontaine the notochord, and the 
nerve-plate too for that matter, are formed by the union in 
the middle-line of two moieties. There is cei-tainly no trace 
of such a process here for the band of dividing cells is con- 
tinuous across the middle-line. Of course iu every structure 
there is an imaginary middle-line, and if anyone chooses to 
say that this baud of dividing cells consists of right and left 
halves Avhich unite together as quickly as they groAv, I shall 
not Avaste time in arguing against such a metaphysical con- 
ception, Avhich is capable neither of pi*oot‘ nor disproof. 
Legros further asserts that the horizontal ridges Avhich he 
finds iu the sides of the archenterou in his abnormal embryo 
mark the limits of the ectodermic and endodermic portions 
of this structure. A glance at fig. He Avill show the futility 
of such a conception ; the histological character of the cells 
forming the upper and the lower portions of the archenterou 
is identical, Avhereas the place Avhere the ectoderm abuts on 
the tissue of the archenteric Avail is clearly seen. But, indeed, 
Legros’s OAvn figures, schematic as they are, refute this con- 
tention, for they shoAV the genuine ectoderm to be Avidely 
different from the so-called ectoderm forming the roof of the 
archenterou. About this stage or a little later the embryo 
escapes from the egg-membrane and becomes a free-SAvimmiug 
larva, propelling itself by its ciliated ectoderm. In figs. 12 a, 
h, c, and d (PI. 3) four sections through a someAvhat older 
embryo are represented, and its smaller diameter, when com- 
pared Avith the embryo Avhich Ave have just considered, should 
be noted (vide the magnifications given in the description 
of the plates). The union of the ectodermic folds above the 
nerve-plate has proceeded forward, and is seen to be complete 
in the most anterior section. In this section Ave see the tAvo 
