40 WHEELER. [Vol. VIII. 



the median line. The upper of these masses is a thickening 

 of the ectoderm distinctly separated on either side from the 

 elements of the same layer by the peculiar character of its 

 cells. These are much smaller than those of the remaining 

 ectoderm and stain more deeply, especially in the inner portions 

 of the layer. The lower mass of cells is entirely cut off from 

 the ectodermal thickening, though its elements are very similar 

 in size and staining qualities. The ectodermal thickening 

 marks the point where the paired labrum is about to appear 

 {cf. Fig. 35). In the next section (Fig. 32), which also 

 passes through the labral region, we again meet with the 

 thickening of the ectoderm. Unlike its portion in the pre- 

 ceding section, it is not bounded below by a curved line, 

 but juts in as a ragged mass of cells, in which it is possible to 

 distinguish a pair of lateral wings and a median projection. 

 The median portion thus proliferated beyond the limits of the 

 ectoderm, is the anterior or oral entoderm centre — the lateral 

 wings I regard as mesodermal. By combining Figs. 32 and 

 33 the flattened mass of cells underlying the ectoderm in the 

 latter section is seen to be the backward continuation of the 

 mesentoderm. Section Fig. 34 shows that this median unpaired 

 mass splits into two masses, one on either side of the mouth. 

 In this paired condition the bands run backwards through the 

 trunk of the embryo. 



Essentially the same condition of the germ-layers in front of 

 the mouth persists till the labrum is definitely formed, as I have 

 observed in a few series of sections. It is difficult to account 

 for the late and intimate union of the mesentoderm with the 

 ectoderm in the labral region, unless we suppose that the 

 blastopore originally extended as far forward as this region and 

 here closed in such a manner that the three layers were not at 

 once separated into ectoderm on the one hand and mesentoderm 

 on the other. It is mainly on this supposition that I take the 

 labral region to coincide with the anterior widening of the 

 blastopore seen in Fig. 2. This widening probably does not 

 coincide with the stomodseum, but lies in front of it, and the 

 definitive mouth is a later formation arising independently from 

 the ectoderm alone. 



