vm AETHEOPODA 183 



abdominal rudiment has anything to do with the closing of the 

 blastopore, and his figures show that this is closed by the union of 

 two flat sheets of endoderm cells, uncovered by ectoderm. Now the 

 lip of the blastopore is a spot where ectoderm passes into endoderm ; 

 it is difficult to imagine that in the process of closing there is a 

 dissolution of this continuity, and the suspicion is aroused that if 

 these stages were worked over by the celloidin- paraffin method 

 different results would be obtained. In all probability the dissolution 

 of continuity is due to the method of section cutting. 



At this same time the cells which formed the endodermic button 

 and which now form the floor of the endodermic sac become more 

 columnar in shape. This increase in size is due to the fact that they 

 begin actively to ingest the yolk granules ; and they continue to do 

 so in successive stages till all the yolk granules, which made up the 

 unsegmented mass in the centre of the egg, are contained in the yolk 

 cells. The endodermic cells increase enormously in length during 

 this process and were termed by the earlier authors the secondary 

 yolk pyramids ; their growth is, however, little advanced in the stage 

 which we are now discussing. ^ 



As the thoracico-abdominal rudiment advances over the blastopore 

 it becomes obviously bilobed, and in the notch between the lobes is 

 seen the last rudiment of the blastopore. In front of this, according 

 to Eeichenbach, i.e. in the bridge which connects the two halves of 

 the rudiment, a new invagination makes its appearance; it is the 

 rudiment of the adult intestine or proctodaeum, which opens by the 

 anus. It is by no means improbable that further investigation 

 would show that the proctodaeum arises just where the last vestige 

 of the blastopore disappeared. 



At the same time the two cephalic lobes have become connected 

 in their hinder region by a curved bridge of columnar cells. This is 

 the rudiment of the labrum or upper lip ; behind it, in a slightly 

 later stage, an invagination appears which will mark the position of 

 the mouth and of the oesophagus (stomodaeum), but of these 

 there is, at this period, no trace. In the streaks of cells connecting 

 the cephalic lobes and thoracico-abdominal rudiments, three outwardly 

 directed, semicircular thickenings are observable, of which the hinder- 

 most pair are the furthest advanced. These are the rudiments of the 

 first three pairs of appendages, viz. the antennules, antennae, 

 and mandibles of the adult. 



The mesoderm when last considered consisted of a small number 

 of large and of a large number of small cells. In this stage the large 

 cells form a mass beneath the thoracico-abdominal rudiment, whilst 

 the smaller have extended and spread all over the surface of the 

 ventral plate and form special aggregations in the cephalic lobes and 

 in the lip rudiment. 



As the rudiments of the appendages become more marked the 

 ventral plate continues to shrink in size and takes on an oval outline. 

 On the median side of each appendage is to be seen a mass of cells 



