the third day of incubation. The transparent zone CAREA 

 PELLUDICA) had a cruciform shape with two long and two 

 short ends; the embryos were situated in the long ends, the 

 posterior ends of which moved apart and the anterior united 

 in a general mass. Both the embryos were equally developed. 

 The spinal plates, containing the foundation of the vertebrae 

 (Pander's PLICAE PRIMITIVAE) were closed, enveloping the 

 spinal cord; the abdominal plates (Wolff's LAMINAE 

 ABDOMINALES) were situated nearly horizontally, i.e. the 

 body of the embryo was still opened from below. The spinal 

 plates passed without discontinuity into the common head, 

 and the spinal cord into the common brain. 



Baer thought it improbable that these partners were at 

 first divided, and then united. In the period up to the 

 third day of incubation, i.e. when the beginning of the 

 process of union could be expected, the embryos cannot move 

 nearer to each other because they are still situated in the 

 convexity of the blastoderm. If it is assumed that the 

 first foundation of the double embryo was single, that the 

 foundation of the head was formed not at its end but in its 

 center, then the formation of the double embryo becomes 

 understandable. The dorsal region of the cephalic end of 

 the embryo grows more quickly than its other parts; therefore 

 in the single embryo the head is curved downwards, and in 

 the double embryo the frontal parts moved away from each 

 other. The cruciform embryonic structures meant that the 

 basis of double-embryo formation occurred in the ovum very 

 early. 



To the question of the origin of double monsters Baer 

 returned in two works published in Petersburg ten years later, 

 The first article^ was very brief and was, in fact, a pre- 

 liminary to the second, which was a vast work. 4 



3. Baer, "Uber doppelleibige Missgeburten," MEM. ACAD. 

 SCIENCES ST.-PETERSB. , VI Ser., v. 3, book 2- 

 BULL. SCIENT.,No. 2 (1835), pp. I-II. 



4. Baer, "Uber doppelleibige Missgeburten oder organische 

 Verdoppelungen in Wirvelthiern," MEM. ACAD. SCIENCES 

 ST. PETERSB.,TV Ser. Sc. nat., v. 4, Zool. et physiol. 

 (1845) , pp. 79-194. 



455 



