172 



from which they come known as the infusorigen, steadily increases 

 in size through division of the elements surrounding the central cell. 

 The cells that have freed themselves from the infusorigen undergo a 

 regular cleavage which results in a second epibolic gastrula. This de- 

 velops into the infusoriform embryo. As the series on either side con- 

 tinually receives new additions at its starting point, we have radiating 

 from the infusorigen two rows of embryos which become older and 

 older the greater the distance from their common source. 



The fully developed infusoriform has a remarkable structure first 

 elucidated byvanBeneden(1876). It is a distinctly bilateral orga- 

 nism nearly as broad as long, rounded anteriorly and somewhat pointed 

 posteriorly. It consists of two portions, what van Beneden has 

 called the "urn'', which corresponds in position with the entoderm cell 

 of the vermiform embryo, and a covering of ectoderm cells. The ecto- 

 derm cells are curiously differentiated. Those occupying the anterior 

 third of the body are smooth and eciliate, the others ("corps ciliaire " 

 of van Beneden) are provided with prowerfuU cilia. Mereover two 

 of the anterior smooth cells are greatly enlarged and each contains a 

 huge refractive body of unknown function. The urn, too, has a rather 

 complicated structure. It consists of a capsule with a lid and contents. 

 This capsule, which is to the contents what the skin of an orangs is 

 to the pulp, consists of two concave cells. The lid which closes the 

 spherical capsule, is composed of four flattened cells, each the qua- 

 drant of a circle. The nuclei of these cells are said to disappear by 

 van Beneden, who also describes certain peculiar details in their 

 structure which I have not been able to recognize as constant features 

 in Dicyema coluber. The contents of the urn consists of a body origi- 

 nally composed of four cells, each the quarter of a sphere, but the four 

 nuclei of these cells soon multiply and steadily decrease in size till the 

 urn is filled with a mass of minute, very deeply staining granules. 



The foregoing is the substance of our knowledge of the structure 

 and life history of the Dicyemidae. In again taking up the study of 

 these neglected parasites I have attempted to find answers to three 

 questions : 



1) Are the nematogenic and rhombogenic individuals two distinct 

 kinds of female, or are they only different phases of the same indivi- 

 dual Dicyemid ? 



2) What is the meaning of the "infusoriform embryo"? 



3) What is the meaning of the "infusorigen"? 



1) Van Beneden answers the first of these questions to the ef- 

 fect that there are two forms of female, comparable to the dimorphic 

 females of the Orthonectidae. This is hardly more than an attempt 



