ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 77 



On the whole it seems to me that then' is every reason for believing that the 

 relations of the egg axis to the embryonic axis are essentially the same in Amphi- 

 oxus and ascidians, that in both the egg axis is postero-dorsal and antero-ventral 

 in direction and that in neither does the neural plate extend more than one-third 

 of the way from the equator to the animal pole (cf. text figs. XXVII-XXXII). 



It' the same axial relations exist in amphibians as in ascidians, the middle 

 of the pigmented hemisphere of the frog's egg does not correspond to the cephalic 

 pole of the embryo but lies ventral to this pole, while the white hemisphere corres- 

 ponds in the main to the dorsal side. This is approximately the orientation which 

 has been maintained by Pfiuger, Roux. Morgan, Kopsch and H. V. Wilson. Kopsch 

 | 1 '.'00), in particular, has shown that the anterior margin of the neural plate lies 

 some distance below the animal pole, and judging from his figures the axial relations 

 in the embryo of the frog must be almost identically like those in the ascidian {cf 

 text tigs. XXXlII-XXXYh 



2. Entrance of Spermatozoon. 



Among ascidians the sperm enters the egg near the vegetal pole ; it then moves 

 to the posterior pole where it meets the egg nucleus, and the sperm amphiaster is 

 formed at right angles to the copulation path. The outer pigmented layer of pro- 

 toplasm collects around the sperm nucleus and moves with it to the posterior pole 

 where the mesodermal crescent is formed. 



In Amphioxus the sperm also enters near the vegetal pole according to Sobotta 

 (1897), hut whether it then moves to the posterior pole and whether there is a 

 collection of superficial protoplasm around the sperm nucleus is unknown. 



In the frog the sperm enters on the posterior side just below the equator and. 

 according to Roux. the point of entrance determines the posterior pole of the embryo. 

 Schultze, on the other hand, thought that the point of entrance lay at the anterior 

 pole, hut since he also with Roux holds that the entrance occurs at the pole oppo- 

 site that at which gastrulation begins, it is evident that this difference with regard 

 to the pole of entrance is only part of the larger difference between these authors as 

 t<> the general orientation of the embryo. The conditions which are found in the 

 ascidian ess closely auree with the orientation of Roux as against that of Schultze. 



CO . < 



In another important respect Roux's observations find a parallel in the ascidian 

 __ : he observed that after fertilization the pigment cap of the fro.i;'- egg shifts so 

 that its margin lies below the equator on the side of the egg where the sperm 

 enters while at the opposite pole it comes to lie above the equator. I believe that 

 this movement of the pigment is comparable to the movements of the layer of 

 yellow protoplasm in the egg of Cynthia. 



?> . Cleavage. 



There are many differences in the cleavage of the egg in these three classes 

 of chordates, hut some fundamental characteristics are essentially similar in all of 

 them. The most important of these is that the cleavage is usually bilaterally 



