48 LILLIE. [Vol. X. 



time of invagination of the shell-gland (Fig. 69, o.p). The 

 bottom of the depression is occupied by numerous small cells 

 very closely appressed, so that the cell-boundaries are seen only 

 with difficulty. In whole embryos the region is marked by 

 the cluster of active nuclei formed here. Flemming applied 

 the name "Mittelschild" to this region in acknowledgment of 

 his ignorance as to its fate; later he conjectured that it might 

 form the oesophagus, but he was so uncertain about it that this 

 can be regarded only as a lucky guess. Rabl at first took it to 

 be the basis of the oesophagus : "Spater aber uberzeugte ich 

 mich dass diese viel weiter oben am Vorderende des Korpers 

 . . . entsteht und sich schon sehr friihzeitig mit den Ento- 

 dermzellen verbindet" (Rabl, I.e., p. 366). Schierholz was the 

 first to clearly recognize its true significance. He applied to 

 it the name "Mundschild," which I shall translate oral plate. 

 The oral plate is destined to form the oesophagus, but it is a 

 very long time before it does so and comes into function. Its 

 place of formation is some distance in front of the anterior end 

 of the blastopore, with which it cannot have had any connec- 

 tion at any previous stage. Later on, however, it moves bodily 

 backwards and meets the ventral plate which at this stage 

 marks the anterior border of the primitive mouth. (In another 

 place I give the details of this process.) 



This brings me to the consideration of the closure of the 

 blastopore and the extension of the ventral plate, tivo processes 

 which go hand /« hand. In its early condition the blastopore 

 has a considerable antero-posterior extent, and is quite wide 

 (PI. V, Figs. 6^ and 6%). Fig. 6^ illustrates the important 

 point that even at this early stage the anterior and posterior 

 lips of the blastopore are bounded by cells of very different 

 appearance. In later stages the distinction becomes much 

 more evident {cf. PI. V, Figs. 69 and 73). The posterior lip of 

 the blastopore is formed by the small cells of the ventral plate 

 (PI. V, Fig. 6^), which was elsewhere (p. 25) shown to be 

 derived from the first somatoblast. The anterior lip is formed 

 by the high columnar cells of the future larval mantle. The 

 mesoderm teloblasts lie just behind the posterior lip. If Fig. 

 69 be compared with Fig. 67 it will be seen that, whereas the 



