74 ORGANIZATION AND CELL-LINEAGE OF 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 he 

 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 winch 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 Amphioxus 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 

 develojnnent of Amphioxus 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. All 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 ahnelt aber die Ascidienentwicklung der des Amphioxus nur in dem einen 

 Stadium, wenn der Urmund geschlossen ist, der Chorda nach hinten auswachst und 

 die Organe der Larva die fiir Wirbelthiere characteristische gegenseitige Lagerung 

 ziegen . . . Bis zu diesem Stadium ist aber die Entwicklung des Amphioxus und 

 der Ascidien so verschiedenen wie mdglich." 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 Amphioxus 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 cell-lineage 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, ft would certainly be surprising if the development of Amphioxus 

 and the ascidians should he found to be more dissimilar than that of annelids 

 and gasteropods. 



