THE PYCNOGONIDS. 19 



appendages, so that I cannot hold this either as a true solution. 

 Beneath these ingrowths appears the pair of thickenings for the 

 brain, which are seen to be continuous across the middle line. 

 On the inner surface of these lobes are seen a few entoblast cells, 

 as shown. 



The next figure, 20, passes through the middle of the brain, 

 but is farther forward than the last (see line 20, Fig. I, Plate III, 

 which indicates the plane of the section). The cells of the ecto- 

 blast seem to pass into those of the brain. Sections still farther 

 forward, where the two lobes are still cut, show the brain thick- 

 enings on each side and separated from each other. We see in 

 surface views of embryos at this stage that the ventral ganglia 

 have appeared, and Fig. 21 is a section through two of these — 

 the first pair. Not only do we see that the ectoblast has greatly 

 thickened in two places on each side the middle line, but the 

 outermost cells of each mass show certain peculiarities. They 

 are much elongated at right angles to the surface, and their outer 

 points come together at the middle point of the surface of the 

 ganglion. The left-hand ganglion in the figure is cut through 

 this central point, and the ganglion on the right a little to one 

 side. The nuclei lie in the inner parts of the cells, and the outer 

 parts of the cells are clear and transparent, while their inner 

 ends are more granular. This difference in the parts of the cells 

 is very constant through the later stages. The cells form a 

 structure which I have called the Ventral Organs. 



In Fig. 22 is a section of a pair of ventral ganglia from an 

 embryo at stage III. Here there is in the middle of each ventral 

 ganglion a wide invagination lined by columnal cells with their 

 clear outer portions turned towards the cavity of the invagina- 

 tion. The nuclei in the cells are larger than those in other parts 

 of the ganglia, and are seen quite often in process of karyokinetic 

 division. The spindles of the dividing nuclei are in some at 

 right angles to the long axis of the cells, and. in others parallel to 

 the axis. A narrow connection of small ectodermal cells runs 

 across the middle line from ganglion to ganglion. 



The next figure, 23, is from an embryo at about stage V. The 

 outer edges of the invagination have turned in until they have 

 nearly met above the cavity of the invagination. In other 

 respects the section is similar to the last. 



