ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. ТТ 
On the whole it seems to me that there 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 zn both the egg axis zs postero-dorsal and antero-ventral 
гп 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). 
If 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 Pflüger, Roux, Morgan, Kopsch and H. V. Wilson. Kopsch 
(1900), 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 (çf. 
text figs. XXXIII-XXXY). 
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), but 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, but 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 
to the general orientation of the embryo. Тһе conditions which are found in the 
ascidian egg closely agree with the orientation of Roux as against that of Schultze. 
In another important respect Roux's observations find a parallel in the ascidian 
egg; he observed that after fertilization the pigment cap of the frog's 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. 
3. Cleavage. 
There are many differences in the cleavage of the egg in these three classes 
of chordates, but some fundamental characteristics are essentially similar in all of 
them. The most important of these is that the cleavage is usually bilaterally 
