74 ORGANIZATION AND CELL-LINEAGE ОҒ ASCIDIAN EGG. 
It is therefore worth while to compare the early development of ascidians 
with that of other primitive chordates in order to see what light may thereby be 
thrown on certain disputed problems. It must, of course, be understood from the 
beginning that such comparison can have the weight only of suggestion ; the prob- 
lems which have been raised in the study of any group can be solved only by the 
further study of that group, but comparisons with other forms may be of great 
service. If evolution be true, if ascidians are genetically related to other chordates, 
then it must be true that their modes of development are related. Whether the 
mode of development of ascidians as compared with Amphzoxus and amphibians is 
palingenetic or coenogenetic is largely a matter of opinion, and need not concern us 
here so only it be granted that there is a relationship between these classes in the 
matter of their development as well as in their later structure. 
Klaatsch (1896) has attempted to elucidate certain disputed points in the 
development of Amphtoxus by a comparison with the ascidians, proceeding upon 
the principle that it is well to reason from the relatively known to the relatively 
unknown, from conclusions in which all agree to questions upon which there is 
diversity of opinion. Samassa (1898), on the other hand, holds that the ascidian 
ontogeny has been so greatly shortened and modified as compared with that of 
Amphioxus that it would be much better to explain the former by the latter than 
the reverse. АП this might be true without destroying the value of comparison, 
but when Samassa further proceeds, as he does in the following sentence, to deny 
that there is any relationship between the two forms except in a single stage, he 
takes away all basis of comparison except for that single stage. He says, p. 20, 
* Nun áhnelt aber die Ascidienentwicklung der des Amphioxus nur in dem einen 
Stadium, wenn der Urmund geschlossen ist, der Chorda nach hinten auswüchst und 
die Organe der Larva die für Wirbelthiere characteristische gegenseitige Lagerung 
ziegen . . . Bis zu diesem Stadium ist aber die Entwicklung des Amphioxus und 
der Ascidien so verschiedenen wie moglich." We have here, if I understand 
Samassa correctly, homologies which are found only in a single stage of the onto- 
geny, which have had no beginnings in homologous parts or processes, have neither 
homological antecedents nor consequents and have therefore arisen de novo. This, 
it seems to me, is the logical conclusion to be drawn from Samassa’s statement, and 
it is one as indefensible on zoological as on philosophical grounds. There are many 
points of resemblance in the early development of Amphzoxus and ascidians, as is 
well known, and such differences as exist are explicable on the general principle of 
evolution through divergent modification. 
The study of the cellineage and early development of a large number of 
annelids and mollusks has shown that in such general matters as the relations of 
the axes of the egg to those of the gastrula and larva, and the origin of the germ 
layers and of specific organs from certain blastomere or regions of the egg, there is 
a high degree of uniformity among members of the same phylum and even among 
related phyla. It would certainly be surprising if the development of Amphioxus 
and the ascidians should be found to be more dissimilar than that of annelids 
and gasteropods. 
AEE MAR See 2 л АК Soh 
