No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 29 



the side. These views are given in order to make the external 

 topography of this important and hitherto misunderstood stage 

 clear. In Fig. 72 the reference line running to Y passes 

 through the blastopore region. The position of the larval and 

 primary mesoblast^ are other points of orientation which make 

 comparison with Figs, 64 and 65 easy. 



Figs. 66 and 6^ (PI. V) are two successive sagittal sections 

 through an embryo of this stage. Fig. 6^ is a true median 

 section, while 66 passes a little to one side of the middle line. 

 The dimensions of the entodermic sac are comparatively insig- 

 nificant, as one would expect from the slight initial size of the 

 entomeres. In this stage the blastopore has a considerable 

 antero-posterior extent (Fig. 6^). 



It will be noticed that the primary mesoblast lies behind, 

 and the larval mesoblast in front of the primitive intestine. 

 There is thus no possible chance of confusing the two struc- 

 tures. They are as distinct in appearance and position as in 

 origin. It was indeed this stage which first convinced me of 

 the twofold origin of the mesoblast in Unio. By following 

 the clue back I arrived at the results already given. 



Previous observers have completely overlooked this stage, 

 or, at least, its anatomy. Goette is the only one who has 

 given a correct account (which is, however, very incomplete) 

 of the origin of the entoderm in the Unionidae, his observations 

 having been made on Anodonta. While he is no doubt right 

 in holding that the invagination of the entoderm occurs at the 

 spot indicated ; still my observations both on whole and sec- 

 tioned larvae of Unio lead me to think that an invagination 

 would be found in an earlier stage of Anodonta at the region 

 in question. 



If I take up the shell-gland now, it is because of its having 

 been more than once (Rabl, Schierholz) confused with the 

 primitive intestine ; also because I have been able to ascertain 

 its cytogeny with certainty, which gives it a place in the first, 

 or cytogenetic division of the paper. The cytogeny of this 

 organ has been already described, so that we can begin 



^ I use the term primary mesoblast for all mesoblast derived from the telo- 

 blasts MM. 



