560 ("RESSAVELL SHEARER. 



tiou of the trocliophore were very briefly studied. He did 

 not observe the origin of the mesoderm cells. 



Hatschek (17), in 1885, studied the development of 

 Eupomatus at Trieste. He supplemented these observa- 

 tions by the examination of a small trochophore found in the 

 Pantano, at Faro, Sicily. The identity of this larva he did 

 not definitely establish. The egg-s studied at Trieste were 

 fertilised by the addition of ripe sperm, and were studied in 

 the living state. Segmentation is equal, and of the spiral 

 type characteristic of many Polycha3ts. In the resultiug 

 blastula the cells from which the germ layers form are already 

 differentiated. The greater part of the lower hemisphere of 

 the blastula produces endoderm. Two cells here larger than 

 the rest give rise to the primitive mesoderm cells, or 

 teloblasts. The region where they lie corresponds to the anal 

 end of the larva. At this time the pre-oral band of cilia 

 makes its appearance as an equatorial circle of cilia. Shortly 

 afterwards the apical cilia appear. The endodermic })arb of 

 the blastula invaginates about nine hours after fertilisation. 

 The two mesoderm cells at the same time move to the interior 

 of the segmentation cavity and detach themselves from their 

 connection with the other cells. The invaginated portion of 

 the endoderm forming the gut then bends towards the anal 

 side of the larva, and fuses with a slight depression of the 

 ectoderm and produces the anus and proctodteum. At the 

 same time the blastopore has become narrowed to a slit, which 

 gradually closes from behind forwards. At the place where 

 the last trace of the blastopore remains the ectoderm invagi- 

 nates and forms the oesophagus. At the same time the two 

 primitive mesoderm cells divide, giving rise to the mesoderm 

 bands, while other cells near the pole-cells of the bands give 

 rise to the head-kidneys; these increase greatly in length and 

 become hollow. The head-kidney then extends from the pole- 

 cells in the region of the anus to the wall of the oesophagus, 

 to which they are attached by a thin protoplasmic strand, 

 while another runs up in the apical region. They open, 

 according to Hatschek, on the exterior on either side of the 



