CHAPTER XI. 

 THE FERTILIZATION AND SEGMENTATION OF THE EGG. 



SECTION 1. The Changes which Accompany Fertilization. 



My most complete series of very young stages is from Salpa hexa- 

 gona, a species which, like Salpa cordiformis, is characterized by the 

 presence of a series of four or five eggs or embryos. Since these are 

 fertilized and undergo their development in succession, a number of 

 stages in the life-history of the embryo may be obtained from each 

 specimen, and as they are all placed in positions which are nearly alike, 

 at least during the early stages, the species presents exceptional advan- 

 tages for study. With only a limited collection of preserved specimens, 

 sections of successive stages in planes which nearly correspond may be 

 obtained from this species with much less difficulty than usually attends 

 the manipulation of such minute eggs. 



Unfortunately, I have had very few eggs of this species, and a still 

 smaller number of specimens of Salpa cordiformis (zonaria). 



Most of my young specimens of Salpa hexagona contained five eggs 

 each, and I think this number is pretty constant, although this is a 

 question which can be easily answered by any one who has an oppor- 

 tunity to examine an abundant supply of fresh specimens. 



As is shown in Plate X, Fig. 10, and also in Plate III, Fig. 1, they 

 form a series along the course of the so-called "oviduct," Fig. 10, X. 

 As no eggs ever pass through this tube, it is not an oviduct in any 

 physiological sense, and I prefer to call it the fertilizing canal, although it 

 may possibly be homologous with the oviduct of other tunicates. The 

 youngest egg, or embryo, is nearest its external opening into the atrium, 

 and the oldest is farthest from it. In Plate X, Fig. 10, A is an unfer- 

 tilized egg, B is one in which segmentation has begun, C is in the stage 

 which is shown in Fig. 8, and D is in the stage shown in Fig. 9. 



If we call that end of the fertilizing duct which opens into the cloaca 

 proximal, there is a distinct lumen proximal to the unfertilized egg, 

 which lies in a pouch-like enlargement or diverticulum in the wall of the 

 duct, as is shown in cross-section in Fig. 1, where almost the whole cavity 



