272 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



cells. He therefore concludes " dass ein wahrer Enterocoelier unter alien 

 Chordaten nicht exist irt." 



I am not able to criticise LwofF's conclusion from the vantage ground 

 of personal investigation of Amphioxus, but his account bears internal 

 evidence of careful and exact observation. He calls attention to a fact, 

 shown by his figures, that, when the mesodermal pouches arise, spindles, 

 if any are present in tlie mesoderm cells, invariably stand vertically to 

 the evaginated layer of cells, foreshadowing an arrangement of the 

 daughter cells in two layers. This is exactly the position ■which the spin- 

 dles take during gastrulation in the mesenchyme cells of Ciona, but in 

 no other cells of the embryo. The form of division in the mesoderm 

 cells of Amphioxus at the period mentioned tends to obliterate the lumen 

 of the mesodermal pouches, a result which, as Lwoff's figures show, actu- 

 ally comes about. A body cavity is formed only secondarily by the 

 moving apart of the mesoderm cells which are arranged in solid masses, 

 the protovertebraj. 



Davidoif ('91) likewise observed in the case of the compound Ascidian, 

 Distaplia, that spindles stand vertically in the cells which give rise to the 

 mesoderm at the time of the separation of the middle germ layer. He 

 believes that the Tunicates can in no sense be regarded as Enterocoelians, 

 and, fin'ther, that the distinction made by the brothers Hertwig between 

 those Metazoa which possess a ^' mesoderm " and those which possess 

 " mesenchyme " is an artificial and unsound one. With these conclu- 

 sions I entirely agree. 



Regarding Rabl's ('89) distinction between "gastral" and "peristo- 

 mal " mesoderm, my observations lead me to the same conclusion as has 

 been expressed by Davidoff, " dass das peristomale Mesoderm der Ascidien 

 sich im weiteren Yerlauf der Entwicklung zum gastralen herausbildet, 

 oder dass das gastrcde Mesoderm urspriinglich peristomales Mesoderm ist." 

 O. Hertwig ('92) draws a similar conclusion regarding Eabl's distinction 

 as applied to the Vertebrates. 



I should also state that both Lwoff" and Wilson ('94) find that the 

 pole mesoderm cells described by Hatschek in the case of Amphioxus 

 do not exist. Certainly nothing of the kind is found among Ascidians. 

 Hence we may conclude that such cells are entirely wanting among 

 Chordates. 



C. Ancestry of the Chordates. 



To determine the phylogenetic relationship of the Chordates to the 

 other groups of Metazoa is a ver}^ difficult problem. Various solutions 



