1902.] CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. 59 



ganglia, etc., from certain cells of the ectoderm. Wilson ('99) has 

 shown that many of the general characteristics mentioned above 

 are applicable also to the cleavage of the Turbellaria. 



With regard to some of these general characteristics it is im- 

 possible at present either to affirm or deny their presence in Tere- 

 bratulina. For example, owing to the difficulty of identifying 

 individual cells I cannot say whether or not the ectoderm is segre- 

 gated in three quartettes. In fact I am wholly unable to recognize 

 quartettes at all after the eight-cell stage. It is certain, however, 

 that the mesoderm is not formed by teleoblastic growth from a sin- 

 gle cell and that the embryo does not grow in length by the teleo- 

 blastic cleavage of two somablasts. Furthermore the cleavage of 

 Terebratulina shows no resemblance to any type of determinate or 

 morphogenetic cleavage which has yet been described, whether 

 among annelids, moUusks, turbellarians, nematodes or ascidians. 

 On the other hand it does resemble in some respects the indeter- 

 minate cleavage of echinoderm.s, Phoronis and ectoproctous Bryozoa. 



2, Mesoderm and Cosio?n. — The gastrulation occurs by typical 

 invagination ; however, this method of gastrulation is found in 

 almost every great group of animals, and therefore no phylogenetic 

 significance can be attributed to it. In the formation of the coelom 

 however the case is somewhat different. The method of mesoderm 

 and coelom formation in Tei-ebratulina is totally unlike that which 

 is found in annelids, mollusks, platyhelminths, nematodes and 

 arthropods, while it shows certain resemblances to chaetognaths and 

 echinoderms. A more detailed comparison shows however that 

 even these resemblances are not very close. 



In echinoderms the enterocoel is formed at the inner end of the 

 archenteron, while the enteron arises from that portion of the arch- 

 enteron nearest the blastopore; in brachiopods the enteron is 

 formed from the inner end of the archenteron, while the enterocoel 

 arises from that part of the archenteron which in echinoderms 

 gives rise to the enteron. It is evident therefore that no real 

 resemblance exists between echinoderms and brachiopods in this 

 respect. 



In chaetognaths the method of coelom formation is more like that 

 in brachiopods— in fact Ko\valevsky supposed that the two were 

 identical— and yet there are important differences here also. In 

 Sagitta, according to both Kowalevsky ('71) and Hertwig ('80), 

 two bilateral folds of the archenteric wall grow into the archen- 



