ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 421 



terminally with the secondary ducts if such exist, and at the external 

 end the ducts unite in a median portion. 



(d) Herold'a duct is formed independently of the generative 

 organs, from the hypodermis ; into it the primary eiferent duct opens. 

 At first simple, it soon forms diverticula, receptacula seminis or 

 glands. In female insects the unpaired duct of Herold communicates 

 with the median portion of the primary efferent ducts. In the males 

 of Collemhola, Campodea, Thysanura, and perhaps in Orthoptera and 

 Hemiptera the communication is similar, but in the other insects the 

 duct of Herold forms paired diverticula, into which the primary ducts 

 open, though apparently in earlier stages the direct communication 

 obtains. 



(e) Oogenesis. — The two types (a) without and (h) with the par- 

 ticipation of yolk-cells are described. In (a) the nuclei and pro- 

 toplasm of the tubule are divided into two layers — the outer, with 

 smaller nuclei forming the epithelium, the inner, with larger spherical 

 nuclei, forming the ova, which are differentiated in order from the 

 efferent duct towards the blind end, where a multiplication occurs in 

 those which deposit their ova singly over a considerable period. In 

 (6) the yolk-cells which become associated with the ova are found 

 either in a single terminal chamber or in several successive chambers, 

 in each of which an ovum is differentiated. The terminal chamber 

 consists at first of the same blastem as the tubules; the internal 

 nuclei become larger, the superficial remain small ; the epithelial 

 layer is separated from the yolk, which may remain undivided or form 

 cells with one or more nuclei. From the protoplasm of the efferent 

 portion, adjacent to the yolk-chamber, a nucleated portion is separated 

 off to form an ovum. The surrounding nucleated protoplasm forms 

 an epithelial envelope. As the portion between the ovum and the 

 terminal chamber increases in length, the first ovum, moving towards 

 the efferent duct, becomes connected with the yolk-chamber by a 

 stalk, and between the two a second ovum is formed, and so on. In 

 Chironomus a single yolk-cell, surrounded by an epithehal layer with 

 small nuclei, occupies each chamber. One of the epithelial nuclei, 

 with some protoplasm, separates itself to form the ovum. The process 

 in Forficula and Diptera is essentially similar. In the others, with 

 multiple yolk-glands, the process is somewhat different. Labidura 

 gigantea is chosen as an instructive case. In the terminal portion 

 only one kind of nuclei at first occurs ; towards the efferent side some 

 become larger — the future yolk-cells ; no epithelial layer is yet pre- 

 sent. A large cell with two nuclei — a yolk-cell nucleus and an egg- 

 cell nucleus — is separated off. At the efferent side an epithelial layer 

 arises, which surrounds the binucleate cell. In the multiple many- 

 celled yolk-glands, e. g. of Bombus, packets of cells are separated off 

 in the terminal portion, each consisting of an ovum and a number of 

 surrounding yolk-cells. In these cases the ovum cannot be regarded 

 as an epithelial cell, and this epithelial character is, in other cases, of 

 subordinate import. The ovum originates neither from epithelial 

 nor from yolk-cells, but from the original blastem of the reproductive 

 rudiment. 



