500 THE AMERICAN A\4rURALIST. [Vol. XXXVIII. 



idea is helped by the fact that the growing embryo soon comes 

 to occupy a position on the yolk strikingly 

 like that of a fish embryo. 



In a word, then, the segmentation of the 

 ovum of Desmognathus is truly holoblastic, 

 but the total cleavage is rather slow in 

 Fig.s.-sectionofamany- appearing, and in the later stages of devel- 

 ceiibiastuia(aboutx8). Qpi^gj^^- y^]]^ ^^^gg becomcs homogeueous 

 by the disappearance of cell walls. 



The Early Development of Dtnophtlus as Compared with that 

 of the Aimelids : James A. Nelson, University of Pennsyl- 

 vania. — The ovum of Dinophilus segments into four cells, or 

 macromeres, each of which gives rise by oblique alternating or 



spiral " cleavages to five generations of smaller cells, or micro- 

 meres. The first three quartettes give rise to ectoderm. The 

 posterior left hand member of the fourth quartette divides 

 bilaterally and equally into two cells which later give rise to the 

 mesoderm bands. The remaining cells give rise to the ento- 

 derm. This method of origin of the three germ layers is 

 identical with that found in the Mollusca and the Polychasta. 

 As in the two former groups the brain is formed from the first 

 quartette, and the greater part of the trunk ectoderm from the 

 posterior cell of the second quartette. In the details of the 

 cleavage there is a striking agreement with that of the poly- 

 chaetous annelids. Cells arising from the first quartette iden- 

 tical in origin with the " primary trochoblasts " of the annelids, 

 together with cells derived from the first and second quartettes, 

 make up a cell row which very probably forms at least a part of 

 the second ciliated band situated on the head of the adult, in a 

 position corresponding with that of the prototroch of the annelid 

 lar\-a. Other details increase this resemblance. The sym- 

 metrical cell pattern known as "the cross "is formed by the 

 same cells and at the same divisions as in the Polych^eta, while 

 m the divisions of the posterior cell of the second quartette 

 bilateral symmetry appears at the same time and in the same 

 cell as in several members of this group. 



Such close agreement in the details of the cleavage, as well 



