The Development of Phascolosoma. 139 



12. Summary. 



Oog-enesis and Spermatogenesis. The oogonia and sper- 

 matog-onia of Fh. vulgare and of Ph. gouldii become detached in 

 clusters from the ovary and the testis, and fall into the coelomic 

 fluid. The oocyte shows no indications of polarity during its period 

 of growth, the nucleus occupying a central position. The spherules 

 of basichromatin, which are scattered through the nucleus, increase 

 in size and abundance. Granules of oxychromatin become collected 

 together into transitory nucleoli. A dense, finely granular layer of 

 the cytoplasm, which immediately surrounds the nucleus, gradually 

 becomes extended towards the surface of the egg. It is to be 

 regarded as an indication that rapid metabolism is going on in the 

 region of the nucleus. Radiating lines of granules pass outward 

 from this layer through the superficial cytoplasm, and are evidently 

 prolonged into the fine protoplasmic processes, which surround the 

 egg until fertilization has occurred. The chitinous vitelline membrane 

 becomes secreted around the proximal extremities of these processes, 

 and thus the pore-canals of the zona radiata (vitelline membrane) 

 are produced. 



The egg of Ph. elongatum Kef. i) is ovoid, like a hen's egg, 

 more opaque than that of Ph. gouldii. which, in turn, contains more 

 yolk than that of Ph. vulgare. 



BreedingSeasonandEgg-layingHabits. The breeding 

 season of Ph. vulgare at Roscofi" extends from the middle of June 

 to the middle of September. That of Ph. gouldii at Newport, R. L 

 begins in the middle of June, and extends to the middle of August, 

 and probably to the middle of September. Ph. gouldii seldom can 

 be induced to lay in the laboratories at AVood's Hole, Mass., though 

 the methods employed for keeping the animals are the same as 

 those that I have found at Newport and at Roscofif to be favorable 



1) This egg is unlike that of the form upon which Selenka's ob- 

 servations at Villefranche were made , and which he probably wrongly 

 identified as I'll, elongatum Kef., for the egg of the southern form is 

 described by Selenka as spherical. The development of the Ph. clon- 

 gaturii of ßoscofif, which is evidently the species which Keferstein (1863) 

 originally described from specimens collected at St. Vaast, on the English 

 Channel, differs also in several respects from that of the Mediterranean 

 form (e. g. in the absence of lateral bristles in the larva). See pag. 124. 



