ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 511 



essentially with those just described. Study of pathological and 

 abnormal specimens appears to show that the large eyes of the two 

 former Planarians have been developed from aggregations of small 

 ones, each consisting of a nervous cell invested by pigment ; such 

 eyes, in fact, appear in some cases as accessory appendages to the 

 main organs. Polycelis nigra has the margin of the anterior end of 

 the body beset with pigmented ocular organs, which are often united 

 together in twos or threes. Their structure differs, however, very 

 widely from that of the eyes of Planaria polychroa. Each eye consists 

 of a homogeneous sphere, invested on its posterior side with a pigment- 

 cup of distinct granules, which is open in front ; in contact with the 

 back of the latter organ is a large transparent hemispherical nucleated 

 cell. The eye appears to be surrounded by gauglion-cells whose 

 nuclei are distinct, but whose exact relations to the eye have not been 

 made out. 



Development of the Orthonectida.* — C. Julin, although agreeing 

 with Metschnikoff in regarding Rhopalura ophiocomce and Intoshia 

 gigas as the male and female forms of the same species, has never 

 been able to detect them both in the same Ophiurid. When an 

 Amphiura squamata infested with males is opened there escape 

 hundreds of individuals in different stages of development. After 

 the first cleavage one of the blastomeres is very much larger than the 

 other, and is more opaque ; this is the ectodermic while the other is 

 the endodermic globule. The former gives rise to as many as fourteen 

 cells before the latter divides at all ; thus there arises a condition of 

 epiboly, where the endodermic cell is ovoid in form and has its long 

 axis parallel to that of the embryo; the enclosed cell now undergoes 

 division, and gives rise to a small cell at either end ; one of these 

 occupies the orifice of the blastopore. These small cells now divide 

 into six and four respectively, and the ectoderm becomes completely 

 ciliated ; as the embryo elongates the small cells increase in length, 

 and becoming fusiform completely envelope the central endodermal 

 mass ; in the adult they form the longitudinally striated fibres. 

 Meantime, the central endodermic cell has divided into a large 

 number of smaller cells, each of which contains a fragment of the 

 primitive endodermal nucleus; each of these gives rise to a sper- 

 matozoon. Although the primordial muscular cells have been given 

 off from it, the central cell still possesses a true membrane, which 

 persists during the whole life of the animal and forms a pouch for the 

 contained spermatozoa. 



While the males are free the embryonic females are connected 

 together by a granular mass (the sporocysts of Giard, plasmodial tubes 

 of Metschnikoff). There would appear to be very great difficulties in 

 the study of their earlier stages owing to this mode of connection. 

 But here also there is ectodermic epiboly, though the endodermic 

 cell divides earlier to give rise to a mass of polyhedral cells, sur- 

 rounded by a layer of cubical non-ciliated cells. Later on these 

 peripheral cells become cylindrical, and still later they form a com- 



* Bull. Sci. Dep. Nord, iv. (1881) pp. 309-18. 



