ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 397 



entrance of a spermatozoon. The author considers that the space between 

 the egg and the vitelline membrane is filled, not with a liquid, but with a 

 gelatinous substance, which shows radiating strife. After the extrusion of 

 the two polar bodies, the true fertilization takes place, as is evidenced by 

 the appearance of a large " aster " in the ovum. 



The segmentation is perfectly regular, despite the quantity of yolk 

 present, and results in a morula. When this has been converted into a 

 blastosphere, cilia appear over all the egg, with the exception of a small area 

 from which the endoderm will be formed. This layer arises by a process 

 of modified invagination. As the cells become slightly inpushed, they 

 increase in size and divide up, so as to form a solid mass. Thus there is 

 no blastopore. A few of the ectoderm cells at the pole opposite the blasto- 

 pore area, i. e. where the endoderm has been formed, are larger than the 

 rest, and have long immobile cilia : this is the commencement of the supra- 

 CBSophageal ganglion. The body-cilia gradually become condensed to a 

 ring round the body. 



A cavity now appears in the endoderm, which will soon communicate 

 with the exterior by a pore — the mouth. The larva elongates in a direction 

 oblique to the axis through the ganglion and blastopore area. The 

 archenteron becomes constricted into oesophagus, stomach, and intestine, 

 which last elongates, attaches itself to the body-wall, and then communi- 

 cates with the exterior. Thus, " neither the oesophagus nor the rectum is 

 formed as a distinct invagination, i. e. there is neither true stomodseum nor 

 proctodseum." The author refers to Hatschek's statement that the intestine 

 of molluscs is an endodermal formation. 



The larva has now become a trochosphere. The preoral band of cilia 

 is carried by large cells, differing histologically from the remaining ectoderm 

 cells, and there is a similar postoral band. There is also a broad band of 

 cilia extending along the median ventral line, the cells bearing them being 

 the rudiments of the nerve-chain. The cells of the ectoderm are crowded 

 with peculiar " wormlike " unicellular glands, which produce a strong 

 secretion, rendering staining difficult. In the ectoderm, also, are numerous 

 muscle-cells, bearing a great resemblance to the epithelio-muscular cells of 

 Coelenterates. 



The histological characters of the different regions of the alimentary 

 tract are described, and the "ciliated ridge" which probably gives rise to 

 the ciliated groove of the adult. The mesoderm consists partly of 

 "mesenchyme," as in Echinoderms, and partly of true mesoderm, as in 

 Annelids. 



The later history of Thalassema is similar to that of Echiurus. The 

 larval segmentation is very conspicuous, especially in the nerve-chain ; 

 this is formed as in Lumhricus. The ventral setae are formed in small 

 mesodermal sacs, which later on acquire an opening through the ectoderm 

 to the exterior. The anal pouches arise as ectodermal invaginations, and 

 funnels gradually arise. Mr. Conn was unable to find any trace of the 

 larval excretory organ described by Hatschek for EcMurus. 



The author, from the developmental history, agrees with the theory 

 that the Gephyrea are extremely modified Annelids, while Polygordius is at 

 the opposite end of the group. He would separate the Sipunculidfe from 

 the Echiuridse ; and BonelUa would have to be placed apart from the 

 latter, owing to its very different mode of development. 



Several theoretical points are touched upon in this paper ; one of them 

 refers to the polar bodies. The author considers that the spermatozoon 

 must exert a certain amount of influence on their formation, for if no 

 spermatozoon enters the egg no polar bodies arc formed. This has been 



