KLEINENBERG ON DEVELOPMENT OF LOPADORHYNCHTJS, 533 



marks the ventral surface, and is not yet connected with the 

 archenteron. The latter is a closed sac, occupying the whole 

 of the interior of the trochosphere, except that a pair of small 

 slits on either side of the stomodoeal invagination represent the 

 remnants of the segmentation cavity. Both ectoderm and 

 endoderm are composed of a single layer of cells. The ecto- 

 derm cells are colourless, their nuclei are distinct, but the cell 

 outlines are difficult to see, and their size and shape vary in 

 different parts of the larva. Those ectoderm cells which bear 

 the cilia of the prototroch are, for instance, much larger than 

 those on the oral and preoral lobes. The endoderm cells are 

 coloured, and their inner ends are stuffed with yolk-granules. 

 Two differentiations can be seen in the ectoderm of the aboral 

 lobe (umbrella, Kl). On the ventral surface, further removed 

 from the aboral pole than from the prototroch, is a pair of 

 areas, where the ectoderm cells are more closely packed 

 together than elsewhere. These are the foundations 1 of the 

 sense-plates of the head. A pair of thickened ectoderm areas 

 can also be seen on the ventral sides of the oral lobe (sub- 

 umbrella, Kl.), the foundations of the ventral plates, and close 

 beneath the stomodaeum an unpaired thickening of the ecto- 

 derm, forming a transient larval organ known as the ventral 

 shield. The further important changes in the larva concern 

 the fate of the ventral plates, and the appearance of the foun- 

 dations of a number of organs on the preoral lobe. On the 

 preoral lobe appear two pairs of prominences in front of the 

 sense-plates, which are presently put into connection with the 

 latter, and the three pairs of structures form a semicircle en- 

 closing a field of thickened ectoderm, the head shield, which 

 occupies nearly the whole ventral surface of the preoral lobe. 

 A single symmetrical pit also makes its appearance towards 

 the aboral pole. At a later period the sense-plates divide to 

 form the foundations of the second pair of permanent antennae 

 anteriorly, and the foundations of the olfactory organs poste- 



1 I use the term " foundations " as the equivalent of the German word 

 A nl age, which cannot be exactly translated into our langunge. The use of 

 the term "rudiment" in this sense is clearly open to objection. 



