DEVELOPMENT OF PLAXULA IN A PLUMULAEIAN HYDEOID. 91 



with no pinnules according- to definition, and Schizotricha 

 with many. Id the present species the formation of the pin- 

 nules tends to be limited to the more proximally placed pinn«, 

 while in Schizotricha it would appear that practically all 

 the pinnas bear them. As a provisional designation, however, 

 the name Schizotricha simplex is proposed. 



n. 



The Development of the Planula. 



The material was fixed in a warm alcoholic solution of cor- 

 rosive sublimate and acetic acid. Several pinnate stems 

 with female gonangia were sectioned in different planes, and 

 the sections were stained wnth Delafield's hasmatoxylin 

 followed by orange. 



Each stem bore gonangia of varying ages, and consequently 

 all the different stages of development would appear to have 

 been observed. Although such was the case, yet the youngest 

 ovum that could be definitely identified as such was already 

 in the endoderm situated at a short distance below the level 

 of the mesial sub-calycine nematophore. 



Presumably, according to August Weismann and fi-om 

 analogy with obsei'vations on other hydroids, the ova first 

 arose in the ectoderm, and then migrated into the endoderm ; 

 but in the present case the young ova were not definitely 

 located in the ectoderm. 



The youngest ovum seen was small, measuring about 14 /u 

 in diameter, and surrounded by ordinary endoderm cells. 

 The presence of the ovum causes a slight swelling to project 

 into the lumen of the internode (PL VI, fig. 1, 0.). 



The ectoderm immediately above the area where the ovum 

 is imbedded in the endoderm early becomes slightly modified 

 in that the cells are more columnar and regular than ordinary 

 ectoderm cells (fig. 1, b. G.). This is the first beginning of 

 the future gonangium. 



The perisarc situated just above {d.j).) has become markedly 



